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.
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