GAppInfo: add a mechanism to query supported content types

This essentially adds an accessor for the MimeType field in desktop files,
to retrieve the list of all mime types supported by an application.
The interface though is part of GAppInfo, so it could be implemented
in the future by other backends.

https://bugzilla.gnome.org/show_bug.cgi?id=674111

docs/reference/gio/gio-sections.txt

@@ -1313,6 +1313,7 @@ g_app_info_set_as_last_used_for_type
 g_app_info_add_supports_type
 g_app_info_can_remove_supports_type
 g_app_info_remove_supports_type
+g_app_info_get_supported_types
 g_app_info_get_all
 g_app_info_get_all_for_type
 g_app_info_get_default_for_type