WindowServer: Add support for fullscreen windows.

Fullscreen windows are rendered alone and above everything else when they
are active, and as part of the regular window stack order when something
else is active.

Currently windows cannot be made fullscreen after-the-fact, but must have
the fullscreen flag included in their CreateWindow message.

It should not possible to interact with the menu, taskbar or window frame
while the active window is fullscreened. :^)

LibGUI/GWindow.cpp

@@ -61,6 +61,7 @@ void GWindow::show()
     request.window.has_alpha_channel = m_has_alpha_channel;
     request.window.modal = m_modal;
     request.window.resizable = m_resizable;
+    request.window.fullscreen = m_fullscreen;
     request.window.opacity = m_opacity_when_windowless;
     request.window.background_color = m_background_color.value();
     request.window.size_increment = m_size_increment;