core: avoid using DBusGMethodInvocation in auth API

While porting to GDBus, use opaque pointers. This allows us to include either a
DBusGMethodInvocation or a GDBusMethodInvocation in the 'context' pointer.

Once fully ported to GDBus, we can safely change it back to make the context be
a GDBusMethodInvocation.

src/mm-auth-request.h

@@ -47,15 +47,18 @@ typedef struct {
 
 GType mm_auth_request_get_type (void);
 
+/* TODO: Change the context gpointer to be a GDBusMethodInvocation when
+ * fully ported to GDBus, and remove all (MMAuthRequestCb) casts */
+
 typedef void (*MMAuthRequestCb) (MMAuthRequest *req,
                                  GObject *owner,
-                                 DBusGMethodInvocation *context,
+                                 gpointer context,
                                  gpointer user_data);
 
 GObject *mm_auth_request_new (GType atype,
                               const char *authorization,
                               GObject *owner,
-                              DBusGMethodInvocation *context,
+                              gpointer context,
                               MMAuthRequestCb callback,
                               gpointer callback_data,
                               GDestroyNotify notify);