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;

LibGUI/GWindow.h

@@ -30,6 +30,9 @@ public:
     bool is_modal() const { return m_modal; }
     void set_modal(bool);
 
+    bool is_fullscreen() const { return m_fullscreen; }
+    void set_fullscreen(bool fullscreen) { m_fullscreen = fullscreen; }
+
     bool is_resizable() const { return m_resizable; }
     void set_resizable(bool resizable) { m_resizable = resizable; }
 
@@ -150,4 +153,5 @@ private:
     bool m_double_buffering_enabled { true };
     bool m_modal { false };
     bool m_resizable { true };
+    bool m_fullscreen { false };
 };