GtkMenuButton: Avoid direct button access

We were getting ourselves in trouble by casting touch events
to GdkEventButton and poking directly at their internals. Instead,
use GdkEvent API to get what we need.

This fixes a crash when using the gear menu in epiphany with
touch. The same crash also occurred in testmenubutton.

gtk/gtkmenubutton.c

@@ -372,11 +372,14 @@ menu_position_side_func (GtkMenu       *menu,
 }
 
 static void
-popup_menu (GtkMenuButton  *menu_button,
-            GdkEventButton *event)
+popup_menu (GtkMenuButton *menu_button,
+            GdkEvent      *event)
 {
   GtkMenuButtonPrivate *priv = menu_button->priv;
   GtkMenuPositionFunc func;
+  GdkDevice *device;
+  guint button;
+  guint32 time;
 
   if (priv->func)
     priv->func (priv->user_data);
@@ -395,14 +398,27 @@ popup_menu (GtkMenuButton  *menu_button,
         break;
   }
 
+  if (event)
+    {
+      device = gdk_event_get_device (event);
+      gdk_event_get_button (event, &button);
+      time = gdk_event_get_time (event);
+    }
+  else
+    {
+      device = NULL;
+      button = 0;
+      time = gtk_get_current_event_time ();
+    }
+
   gtk_menu_popup_for_device (GTK_MENU (priv->menu),
-                             event ? event->device : NULL,
+                             device,
                              NULL, NULL,
                              func,
                              GTK_WIDGET (menu_button),
                              NULL,
-                             event ? event->button : 0,
-                             event ? event->time : gtk_get_current_event_time ());
+                             button,
+                             time);
 }
 
 static void
@@ -418,9 +434,7 @@ gtk_menu_button_clicked (GtkButton *button)
 
       event = gtk_get_current_event ();
 
-      popup_menu (menu_button,
-                  (event && event->type != GDK_BUTTON_RELEASE) ?
-                  (GdkEventButton *) event : NULL);
+      popup_menu (menu_button, event);
 
       if (!event ||
           event->type == GDK_KEY_PRESS ||