g_app_info_launch_default_for_uri: don't use GFile if we don't have to

g_app_info_launch_default_for_uri: don't use GFile if we don't have to

GFile doesn't handle some "real" URIs, so check if there's a default
handler for the URI scheme first, and only use g_file_new_for_uri()
and g_file_query_default_handler() if not. Eg, this fixes the case of
opening http URIs with "%2F" in the path.

https://bugzilla.gnome.org/show_bug.cgi?id=666386