Introduce MouseEventHandler

Still need to give elements the ability to re-render their parent view. Once that is in place, I think we can implement hoverable close tab buttons.

gpui/src/platform/event.rs

@@ -21,4 +21,7 @@ pub enum Event {
     LeftMouseDragged {
         position: Vector2F,
     },
+    MouseMoved {
+        position: Vector2F,
+    },
 }

gpui/src/platform/mac/event.rs

@@ -108,6 +108,12 @@ impl Event {
                 ),
                 precise: native_event.hasPreciseScrollingDeltas() == YES,
             }),
+            NSEventType::NSMouseMoved => window_height.map(|window_height| Self::MouseMoved {
+                position: vec2f(
+                    native_event.locationInWindow().x as f32,
+                    window_height - native_event.locationInWindow().y as f32,
+                ),
+            }),
             _ => None,
         }
     }