Work toward turning GdkDisplayManager into a backend singleton
This commit hides the GdkDisplayManager instance and class structs, adds vfuncs for listing displays, opening displays, and getting and setting the default display. The X11 backend has a derived GdkDisplayManagerX11. The gdk_display_manager_get() function is responsible for deciding on which of the compiled in backends to use. Currently, it consults the GDK_BACKEND environment variable and falls back to x11.