Make GdkAppLaunchContext display-dependent

Make GdkAppLaunchContext display-dependent

Add a GdkDisplay::get_app_launch_context vfunc, and a
gdk_display_get_app_launch_context that for X11 returns a subclass.
For win32 and quartz, the implementations were trivial, so we
just return a new GdkAppLaunchContext without subclassing. Since
the type of the context now depends on the display,
gdk_app_launch_context_set_display is deprecated.