This patch changes all uses of GDK_DEPRECATED(_FOR) in gdk headers
by the versioned variants, GDK_DEPRECATED_IN_3_x(_FOR). At the same
time, we add GDK_AVAILABLE_IN_3_x annotations for all API additions
in 3.2 and 3.4.
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.