WindowServer: Add WindowFrame::invalidate_menubar() and use it

Clean up a FIXME about overly aggressive invalidation.

Userland/Services/WindowServer/Window.cpp

@@ -1264,8 +1264,7 @@ void Window::invalidate_menubar()
 {
     if (!m_should_show_menubar || !menubar())
         return;
-    // FIXME: This invalidates way more than the menubar!
+    frame().invalidate_menubar();
-    frame().invalidate();
 }
 
 void Window::set_modified(bool modified)

Userland/Services/WindowServer/WindowFrame.cpp

@@ -585,6 +585,11 @@ void WindowFrame::invalidate()
     m_window.invalidate(true, true);
 }
 
+void WindowFrame::invalidate_menubar()
+{
+    invalidate(menubar_rect());
+}
+
 void WindowFrame::invalidate(Gfx::IntRect relative_rect)
 {
     auto frame_rect = rect();

Userland/Services/WindowServer/WindowFrame.h

@@ -70,6 +70,7 @@ public:
 
     void window_rect_changed(const Gfx::IntRect& old_rect, const Gfx::IntRect& new_rect);
     void invalidate_titlebar();
+    void invalidate_menubar();
     void invalidate(Gfx::IntRect relative_rect);
     void invalidate();