appinfo: add g_app_info_set_as_last_used_for_type()

This commit also changes (maintaining compatibility) the way
user-specified default applications are stored (as in, those for which
g_app_info_set_as_default_for_type() has been called.

We now store the default application for a content type in a new group
in the mimeapps.list keyfile, and "Added Associations" tracks only the
applications that have been added by the user, following a
most-recently-used first order.

This is useful in GtkAppChooser-like widgets to pre-select the last used
application when constructing a widget.

https://bugzilla.gnome.org/show_bug.cgi?id=636311

gio/gappinfo.c

@@ -308,6 +308,33 @@ g_app_info_set_as_default_for_type (GAppInfo    *appinfo,
   return (* iface->set_as_default_for_type) (appinfo, content_type, error);
 }
 
+/**
+ * g_app_info_set_as_last_used_for_type:
+ * @appinfo: a #GAppInfo.
+ * @content_type: the content type.
+ * @error: a #GError.
+ * 
+ * Sets the application as the last used application for a given type.
+ * This will make the application appear as first in the list returned by
+ * #g_app_info_get_recommended_for_type, regardless of the default application
+ * for that content type.
+ *
+ * Returns: %TRUE on success, %FALSE on error.
+ **/
+gboolean
+g_app_info_set_as_last_used_for_type (GAppInfo    *appinfo,
+                                      const char  *content_type,
+                                      GError     **error)
+{
+  GAppInfoIface *iface;
+  
+  g_return_val_if_fail (G_IS_APP_INFO (appinfo), FALSE);
+  g_return_val_if_fail (content_type != NULL, FALSE);
+
+  iface = G_APP_INFO_GET_IFACE (appinfo);
+
+  return (* iface->set_as_last_used_for_type) (appinfo, content_type, error);
+}
 
 /**
  * g_app_info_set_as_default_for_extension: