Minor refactoring of mouse event types

* Move some types into the `msg` crate so they can be shared more.
* Use MouseEventType instead of duplicating it in other enums.

components/msg/constellation_msg.rs

@@ -307,6 +307,24 @@ pub enum ScriptMsg {
     ViewportConstrained(PipelineId, ViewportConstraints),
 }
 
+#[derive(Deserialize, HeapSizeOf, Serialize)]
+pub enum MouseEventType {
+    Click,
+    MouseDown,
+    MouseUp,
+}
+
+/// The mouse button involved in the event.
+#[derive(Clone, Copy, Debug, Deserialize, Serialize)]
+pub enum MouseButton {
+    /// The left mouse button.
+    Left,
+    /// The middle mouse button.
+    Middle,
+    /// The right mouse button.
+    Right,
+}
+
 /// Messages from the paint task to the constellation.
 #[derive(Deserialize, Serialize)]
 pub enum PaintMsg {