compiler: Improve diagnostics when using layout properties

Clarify that for example rowspan and colspan are to be used in a cell. Case in point:

```
GridLayout {
    rowspan: 2;
```

shouldn't say that "rowspan" can't be used in a GridLayout when it's used in a grid layout :)

Author: tronical

internal/compiler/passes/lower_layout.rs

@@ -856,12 +856,15 @@ fn check_no_layout_properties(
         if parent_layout_type.as_deref() != Some("GridLayout")
             && matches!(prop.as_ref(), "col" | "row" | "colspan" | "rowspan")
         {
-            diag.push_error(format!("{prop} used outside of a GridLayout"), &*expr.borrow());
+            diag.push_error(format!("{prop} used outside of a GridLayout's cell"), &*expr.borrow());
         }
         if parent_layout_type.as_deref() != Some("Dialog")
             && matches!(prop.as_ref(), "dialog-button-role")
         {
-            diag.push_error(format!("{prop} used outside of a Dialog"), &*expr.borrow());
+            diag.push_error(
+                format!("{prop} used outside of a Dialog's direct child"),
+                &*expr.borrow(),
+            );
         }
         if layout_type.is_none()
             && matches!(

internal/compiler/tests/syntax/basic/dialog.slint

@@ -18,9 +18,9 @@ MyDiag2 := Dialog {
     StandardButton {
         kind: cancel;
         col: 42;
-//           ^error{col used outside of a GridLayout}
+//           ^error{col used outside of a GridLayout's cell}
         rowspan: 2;
-//               ^error{rowspan used outside of a GridLayout}
+//               ^error{rowspan used outside of a GridLayout's cell}
     }
 }
 
@@ -47,7 +47,7 @@ MyDialog4 := Dialog {
 //                                  ^error{The `dialog-button-role` property must be known at compile-time}
     Rectangle {
         Rectangle { dialog-button-role: accept; }
-//                                      ^error{dialog-button-role used outside of a Dialog}
+//                                      ^error{dialog-button-role used outside of a Dialog's direct child}
     }
 
     init => {

internal/compiler/tests/syntax/basic/layout2.slint

@@ -41,17 +41,17 @@ export X := Rectangle {
             row: 200000; // that's actually bigger than 65535
 //               ^error{'row' must be a positive integer}
             Rectangle { row: 3; }
-//                           ^error{row used outside of a GridLayout}
+//                           ^error{row used outside of a GridLayout's cell}
         }
     }
 
     Text { colspan: 3; }
-//                  ^error{colspan used outside of a GridLayout}
+//                  ^error{colspan used outside of a GridLayout's cell}
     col: 3;
-//       ^error{col used outside of a GridLayout}
+//       ^error{col used outside of a GridLayout's cell}
 
     HorizontalLayout {
         col: 3;
-//           ^error{col used outside of a GridLayout}
+//           ^error{col used outside of a GridLayout's cell}
     }
 }