Reorganize for more nesting.

This removes the majority of structs from the crate root and puts all the controls in the controls module. It also adds a prelude, designed to be the only import from this library other than the specific controls used.

Author: NoraCodes

ui/examples/basic.rs

@@ -1,6 +1,6 @@
 extern crate ui;
-use std::rc::Rc;
-use ui::{UI, Window, BoxControl, Button};
+use ui::prelude::*;
+use ui::controls::{BoxControl, Button};
 
 fn main() {
     // Start up the UI toolkit

ui/examples/controlgallery.rs

@@ -2,10 +2,9 @@
 
 extern crate ui;
 
-use std::rc::Rc;
-
-use ui::{UI, BoxControl, Button, Checkbox, ColorButton, Combobox, EditableCombobox, DateTimePicker,
-         Entry, FontButton, Group, Label, Menu, MenuItem, ProgressBar, RadioButtons, Separator, Slider, Spinbox, Tab, Window};
+use ui::prelude::*;
+use ui::controls::*;
+use ui::menus::{Menu, MenuItem};
 
 fn run(ui: Rc<UI>) {
     let menu = Menu::new("File");