Refactor|Client: ClientWindow implemented, added GuiRootWidget

The ClientWindow is the highest-level window class that manages the UI
widgets in the window.

GuiRootWidget is the root widget that is used inside a ClientWindow.
libgui: Cleanup and post-refactoring fixes
Refactor|Client: Use new ClientWindow class, WindowSystem methods

Moved window management methods to WindowSystem.
Fixed|Server: DisplayMode is not needed in the server
All Games: Link game plugins with libgui for GUI features (DDKEY)
Refactor|Fixed|Client: Client window GL init