Auto merge of #512 - Hupka:master, r=jrmuizel

Fixes #511: Add`NSPanel` trait to create NSPanels

Intention for PR described here: #511.

I didn't extend any of the examples to include `NSPanel` because I think that's not really worth it. Instead, here is an adapted version of the `hello_world.rs` example that prints out responses of the API for both, `NSWindow` and `NSPanel`.

  ```Rust
extern crate cocoa;

use cocoa::appkit::{
    NSApp, NSApplication, NSApplicationActivateIgnoringOtherApps,
    NSApplicationActivationPolicyRegular, NSBackingStoreBuffered, NSMenu, NSMenuItem, NSPanel,
    NSRunningApplication, NSWindow, NSWindowStyleMask,
};
use cocoa::base::{nil, selector, NO};
use cocoa::foundation::{NSAutoreleasePool, NSPoint, NSProcessInfo, NSRect, NSSize, NSString};

fn main() {
    unsafe {
        let _pool = NSAutoreleasePool::new(nil);

        let app = NSApp();
        app.setActivationPolicy_(NSApplicationActivationPolicyRegular);

        // create Menu Bar
        let menubar = NSMenu::new(nil).autorelease();
        let app_menu_item = NSMenuItem::new(nil).autorelease();
        menubar.addItem_(app_menu_item);
        app.setMainMenu_(menubar);

        // create Application menu
        let app_menu = NSMenu::new(nil).autorelease();
        let quit_prefix = NSString::alloc(nil).init_str("Quit ");
        let quit_title =
            quit_prefix.stringByAppendingString_(NSProcessInfo::processInfo(nil).processName());
        let quit_action = selector("terminate:");
        let quit_key = NSString::alloc(nil).init_str("q");
        let quit_item = NSMenuItem::alloc(nil)
            .initWithTitle_action_keyEquivalent_(quit_title, quit_action, quit_key)
            .autorelease();
        app_menu.addItem_(quit_item);
        app_menu_item.setSubmenu_(app_menu);

        // create Window
        let window = NSWindow::alloc(nil)
            .initWithContentRect_styleMask_backing_defer_(
                NSRect::new(NSPoint::new(0., 0.), NSSize::new(200., 200.)),
                NSWindowStyleMask::NSTitledWindowMask,
                NSBackingStoreBuffered,
                NO,
            )
            .autorelease();
        window.cascadeTopLeftFromPoint_(NSPoint::new(20., 20.));
        window.center();
        let title = NSString::alloc(nil).init_str("Hello World!");
        window.setTitle_(title);
        window.makeKeyAndOrderFront_(nil);

        // create Panel
        let panel = NSPanel::alloc(nil)
            .initWithContentRect_styleMask_backing_defer_(
                NSRect::new(NSPoint::new(0., 0.), NSSize::new(200., 200.)),
                NSWindowStyleMask::NSTitledWindowMask,
                NSBackingStoreBuffered,
                NO,
            )
            .autorelease();
        panel.cascadeTopLeftFromPoint_(NSPoint::new(20., 20.));
        panel.center();
        let title = NSString::alloc(nil).init_str("Hello PANEL!");
        panel.setTitle_(title);
        panel.makeKeyAndOrderFront_(nil);

        // Specific behavior of NSWindow & NSPanel
        println!(
            "NSWindow can become main window: {}",
            window.canBecomeMainWindow()
        ); // returns true
        println!(
            "NSPanel can become main window? {}",
            panel.canBecomeMainWindow()
        ); // returns false

        // Test methods of NSPanel Trait
        println!(
            "BEFORE setting it: NSPanel is a floating panel: {}",
            panel.floatingPanel()
        ); // returns false
        println!(
            "BEFORE setting it: NSPanel becomes key only when needed: {}",
            panel.becomesKeyOnlyIfNeeded()
        ); // returns false

        panel.setFloatingPanel(true);
        panel.setBecomesKeyOnlyIfNeeded(true);

        println!(
            "AFTER: NSPanel is a floating panel: {}",
            panel.floatingPanel()
        ); // returns true
        println!(
            "AFTER: NSPanel becomes key only when needed: {}",
            panel.becomesKeyOnlyIfNeeded()
        ); // returns true

        let current_app = NSRunningApplication::currentApplication(nil);
        current_app.activateWithOptions_(NSApplicationActivateIgnoringOtherApps);
        app.run();
    }
}
```

Author: bors-servo

cocoa/src/appkit.rs

@@ -1802,6 +1802,44 @@ impl NSWindow for id {
     // TODO: Constraint-Based Layouts
 }
 
+pub trait NSPanel: Sized {
+    unsafe fn alloc(_: Self) -> id {
+        msg_send![class!(NSPanel), alloc]
+    }
+
+    // NSPanel subclasses NSWindow, hence we only add the added methods
+    // https://developer.apple.com/documentation/appkit/nspanel
+    unsafe fn setBecomesKeyOnlyIfNeeded(self, becomesKeyOnlyIfNeeded: BOOL);
+    unsafe fn becomesKeyOnlyIfNeeded(self) -> BOOL;
+    unsafe fn setFloatingPanel(self, floatingPanel: BOOL);
+    unsafe fn floatingPanel(self) -> BOOL;
+    unsafe fn setWorksWhenModal(self, worksWithPanel: BOOL);
+}
+
+impl NSPanel for id {
+    // NSPanel subclasses NSWindow, hence we only add the added methods
+    // https://developer.apple.com/documentation/appkit/nspanel
+    unsafe fn setBecomesKeyOnlyIfNeeded(self, becomesKeyOnlyIfNeeded: BOOL) {
+        msg_send![self, setBecomesKeyOnlyIfNeeded: becomesKeyOnlyIfNeeded]
+    }
+
+    unsafe fn becomesKeyOnlyIfNeeded(self) -> BOOL {
+        msg_send![self, becomesKeyOnlyIfNeeded]
+    }
+
+    unsafe fn setFloatingPanel(self, floatingPanel: BOOL) {
+        msg_send![self, setFloatingPanel: floatingPanel]
+    }
+
+    unsafe fn floatingPanel(self) -> BOOL {
+        msg_send![self, isFloatingPanel]
+    }
+
+    unsafe fn setWorksWhenModal(self, worksWhenModal: BOOL) {
+        msg_send![self, setWorksWhenModal: worksWhenModal]
+    }
+}
+
 #[repr(i64)]
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum NSModalResponse {