LibGUI: Add Desktop::{taskbar,menubar}_height()

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/852770687a4
Pull-request: https://github.com/SerenityOS/serenity/pull/3118
Issue: https://github.com/SerenityOS/serenity/issues/3115
Reviewed-by: https://github.com/awesomekling

Libraries/LibGUI/Desktop.h

@@ -46,6 +46,10 @@ public:
     bool set_wallpaper(const StringView& path);
 
     Gfx::IntRect rect() const { return m_rect; }
+
+    int taskbar_height() const { return 28; }
+    int menubar_height() const { return 19; }
+
     void did_receive_screen_rect(Badge<WindowServerConnection>, const Gfx::IntRect&);
 
     Function<void(const Gfx::IntRect&)> on_rect_change;