view: add :will_add_event vfunc and stub methods

Note: this commit probably won't build, but this is the right order of
keeping history.

src/gcal-month-view.c

@@ -133,6 +133,9 @@ static GtkWidget*     gcal_month_view_get_by_uuid           (GcalView       *vie
 
 static void           gcal_month_view_clear                 (GcalView       *view);
 
+static gboolean       gcal_month_view_will_add_event        (GcalView        *view,
+                                                             GcalEventWidget *event);
+
 G_DEFINE_TYPE_WITH_CODE (GcalMonthView,
                          gcal_month_view,
                          GTK_TYPE_CONTAINER,
@@ -214,6 +217,8 @@ gcal_view_interface_init (GcalViewIface *iface)
   iface->draw_event = gcal_month_view_draw_event;
   iface->get_by_uuid = gcal_month_view_get_by_uuid;
   iface->clear = gcal_month_view_clear;
+
+  iface->will_add_event = gcal_month_view_will_add_event;
 }
 
 static void
@@ -1295,6 +1300,13 @@ gcal_month_view_clear (GcalView *view)
                          (GtkCallback) gtk_widget_destroy, NULL);
 }
 
+static gboolean
+gcal_month_view_will_add_event (GcalView        *view,
+                                GcalEventWidget *event)
+{
+  ;
+}
+
 /* Public API */
 /**
  * gcal_month_view_new: