The g_app_info_launch_uris_async() and g_app_info_launch_uris_finish()
functions are crucial to fix g_app_info_launch_default_for_uri_async()
to be really asynchronous.

This patch also adds GDesktopAppInfo implementation of that vfuncs.
The implementation may still use some synchronous calls to local MIME DB.

#1347
#1249