gio: Add GTlsInteraction interaction method invocation guarantees

* Add 'invoke' style method, which can be used to call an interaction
   from any thread. The interaction will be run in the appropriate
   #GMainContext
 * Sync methods can be called whether main loop is running or not.
 * Derived classes can choose to implement only sync or async
   interaction method, and the invoke method will fill in the blanks.
 * Documentation for the above.
 * Tests for the above.

https://bugzilla.gnome.org/show_bug.cgi?id=657567

gio/gtlsinteraction.h

@@ -43,17 +43,17 @@ typedef struct _GTlsInteractionPrivate GTlsInteractionPrivate;
 
 struct _GTlsInteraction
 {
+  /*< private >*/
   GObject parent_instance;
-
   GTlsInteractionPrivate *priv;
 };
 
 struct _GTlsInteractionClass
 {
+  /*< private >*/
   GObjectClass parent_class;
 
-  /* virtual methods: */
-
+  /*< public >*/
   GTlsInteractionResult  (* ask_password)        (GTlsInteraction    *interaction,
                                                   GTlsPassword       *password,
                                                   GCancellable       *cancellable,
@@ -76,6 +76,12 @@ struct _GTlsInteractionClass
 
 GType                  g_tls_interaction_get_type            (void) G_GNUC_CONST;
 
+GTlsInteractionResult  g_tls_interaction_invoke_ask_password (GTlsInteraction    *interaction,
+                                                              GTlsPassword       *password,
+                                                              GCancellable       *cancellable,
+                                                              GError            **error);
+
+
 GTlsInteractionResult  g_tls_interaction_ask_password        (GTlsInteraction    *interaction,
                                                               GTlsPassword       *password,
                                                               GCancellable       *cancellable,