gdk: Implement GInitable on GdkDisplayManager

gdk: Implement GInitable on GdkDisplayManager

Add GInitable interface with a default implementation that always
succeeds. This allows backends to override the GInitable implementation
and add their own checks to determine if the backend can be loaded.  If
a backend cannot be loaded, GDK can attempt to load the next available
backend.

Since backends may need to read any relevant options (such as the
display flag) to determine if they can be created successfully, this
patch also removes calls that attempt to create the display manager
before the options have been parsed.

https://bugzilla.gnome.org/show_bug.cgi?id=694465