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/linux/wayland/window.rs

@@ -31,8 +31,8 @@ use crate::{
     AnyWindowHandle, Bounds, Decorations, Globals, GpuSpecs, Modifiers, Output, Pixels,
     PlatformDisplay, PlatformInput, Point, PromptButton, PromptLevel, RequestFrameOptions,
     ResizeEdge, ScaledPixels, Size, Tiling, WaylandClientStatePtr, WindowAppearance,
-    WindowBackgroundAppearance, WindowBounds, WindowControls, WindowDecorations, WindowParams, px,
-    size,
+    WindowBackgroundAppearance, WindowBounds, WindowControlArea, WindowControls, WindowDecorations,
+    WindowParams, px, size,
 };
 
 #[derive(Default)]
@@ -978,6 +978,10 @@ impl PlatformWindow for WaylandWindow {
         self.0.callbacks.borrow_mut().close = 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.callbacks.borrow_mut().appearance_changed = Some(callback);
     }

crates/gpui/src/platform/linux/x11/window.rs

@@ -5,8 +5,8 @@ use crate::{
     AnyWindowHandle, Bounds, Decorations, DevicePixels, ForegroundExecutor, GpuSpecs, Modifiers,
     Pixels, PlatformAtlas, PlatformDisplay, PlatformInput, PlatformInputHandler, PlatformWindow,
     Point, PromptButton, PromptLevel, RequestFrameOptions, ResizeEdge, ScaledPixels, Scene, Size,
-    Tiling, WindowAppearance, WindowBackgroundAppearance, WindowBounds, WindowDecorations,
-    WindowKind, WindowParams, X11ClientStatePtr, px, size,
+    Tiling, WindowAppearance, WindowBackgroundAppearance, WindowBounds, WindowControlArea,
+    WindowDecorations, WindowKind, WindowParams, X11ClientStatePtr, px, size,
 };
 
 use blade_graphics as gpu;
@@ -1408,6 +1408,10 @@ impl PlatformWindow for X11Window {
         self.0.callbacks.borrow_mut().close = 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.callbacks.borrow_mut().appearance_changed = Some(callback);
     }