WindowServer+CPUGraph: Make menu applets be "regular" windows

Instead of implementing menu applets as their own thing, they are now
WSWindows of WSWindowType::MenuApplet.

This makes it much easier to work with them on the client side, since
you can just create a GWindow with the right type and you're in the
menubar doing applet stuff :^)

Libraries/LibGUI/GWindowType.h

@@ -9,4 +9,5 @@ enum class GWindowType {
     Taskbar,
     Tooltip,
     Menubar,
+    MenuApplet,
 };