WindowServer: Add message to notify clients of applet area resize

Applets and windows would like to be able to know when the applet
area has been resized. For example, this happens asynchronously after
an applet has been resized, so we cannot then rely on the applet area
position synchronously after resizing. This adds a new message
applet_area_rect_changed and associated Event AppletAreaRectChange,
and the appropriate virtual functions.

Userland/Libraries/LibGUI/WindowServerConnection.cpp

@@ -321,6 +321,13 @@ void WindowServerConnection::screen_rects_changed(Vector<Gfx::IntRect> const& re
     });
 }
 
+void WindowServerConnection::applet_area_rect_changed(Gfx::IntRect const& rect)
+{
+    Window::for_each_window({}, [&](auto& window) {
+        Core::EventLoop::current().post_event(window, make<AppletAreaRectChangeEvent>(rect));
+    });
+}
+
 void WindowServerConnection::set_wallpaper_finished(bool)
 {
     // This is handled manually by Desktop::set_wallpaper().