Taskbar: Update and clear taskbar button rectangles

We need to update all button rectangles when the layout changed.
We also need to clear the taskbar button rectangle when we
remove a modal window button, so that WindowServer uses the
parent's taskbar button rectangle for the minimize animation.

Services/Taskbar/TaskbarButton.h

@@ -34,6 +34,9 @@ class TaskbarButton final : public GUI::Button {
 public:
     virtual ~TaskbarButton() override;
 
+    void update_taskbar_rect();
+    void clear_taskbar_rect();
+
 private:
     explicit TaskbarButton(const WindowIdentifier&);