gpui: Implement dynamic window control elements (#30828)

Allows setting element as window control elements which consist of
`Drag`, `Close`, `Max`, or `Min`. This allows you to implement
dynamically sized elements that control the platform window, this is
used for areas such as the title bar. Currently only implemented for
Windows.

Release Notes:

- N/A

crates/gpui/src/platform/mac/window.rs

@@ -4,8 +4,8 @@ use crate::{
     KeyDownEvent, Keystroke, Modifiers, ModifiersChangedEvent, MouseButton, MouseDownEvent,
     MouseMoveEvent, MouseUpEvent, Pixels, PlatformAtlas, PlatformDisplay, PlatformInput,
     PlatformWindow, Point, PromptButton, PromptLevel, RequestFrameOptions, ScaledPixels, Size,
-    Timer, WindowAppearance, WindowBackgroundAppearance, WindowBounds, WindowKind, WindowParams,
-    platform::PlatformInputHandler, point, px, size,
+    Timer, WindowAppearance, WindowBackgroundAppearance, WindowBounds, WindowControlArea,
+    WindowKind, WindowParams, platform::PlatformInputHandler, point, px, size,
 };
 use block::ConcreteBlock;
 use cocoa::{
@@ -1146,6 +1146,10 @@ impl PlatformWindow for MacWindow {
         self.0.as_ref().lock().close_callback = Some(callback);
     }
 
+    fn on_hit_test_window_control(&self, _callback: Box<dyn FnMut() -> Option<WindowControlArea>>) {
+        unimplemented!()
+    }
+
     fn on_appearance_changed(&self, callback: Box<dyn FnMut()>) {
         self.0.lock().appearance_changed_callback = Some(callback);
     }