Work toward turning GdkDisplayManager into a backend singleton

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.