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.rs

@@ -36,7 +36,7 @@ use crate::{
     ForegroundExecutor, GlyphId, GpuSpecs, ImageSource, Keymap, LineLayout, Pixels, PlatformInput,
     Point, RenderGlyphParams, RenderImage, RenderImageParams, RenderSvgParams, ScaledPixels, Scene,
     ShapedGlyph, ShapedRun, SharedString, Size, SvgRenderer, SvgSize, Task, TaskLabel, Window,
-    hash, point, px, size,
+    WindowControlArea, hash, point, px, size,
 };
 use anyhow::Result;
 use async_task::Runnable;
@@ -436,6 +436,7 @@ pub(crate) trait PlatformWindow: HasWindowHandle + HasDisplayHandle {
     fn on_resize(&self, callback: Box<dyn FnMut(Size<Pixels>, f32)>);
     fn on_moved(&self, callback: Box<dyn FnMut()>);
     fn on_should_close(&self, callback: Box<dyn FnMut() -> bool>);
+    fn on_hit_test_window_control(&self, callback: Box<dyn FnMut() -> Option<WindowControlArea>>);
     fn on_close(&self, callback: Box<dyn FnOnce()>);
     fn on_appearance_changed(&self, callback: Box<dyn FnMut()>);
     fn draw(&self, scene: &Scene);