Bug 659406 - Abstract what triggers a context menu

Add gdk_event_triggers_context_menu(), using the new modifier
abstraction API. Remove _gtk_button_event_triggers_context_menu()
and port all callers.

gdk/gdkevents.c

@@ -1448,6 +1448,56 @@ gdk_event_request_motions (const GdkEventMotion *event)
     }
 }
 
+/**
+ * gdk_event_triggers_context_menu:
+ * @event: a #GdkEvent, currently only button events are meaningful values
+ *
+ * This function returns whether a #GdkEventButton should trigger a
+ * context menu, according to platform conventions. The right mouse
+ * button always triggers context menus. Additionally, if
+ * gdk_keymap_get_modifier_mask() returns a non-0 mask for
+ * %GDK_MODIFIER_INTENT_CONTEXT_MENU, then the left mouse button will
+ * also trigger a context menu if this modifier is pressed.
+ *
+ * This function should always be used instead of simply checking for
+ * event->button == 3.
+ *
+ * Returns: %TRUE if the event should trigger a context menu.
+ *
+ * Since: 3.4
+ **/
+gboolean
+gdk_event_triggers_context_menu (const GdkEvent *event)
+{
+  g_return_val_if_fail (event != NULL, FALSE);
+
+  if (event->type == GDK_BUTTON_PRESS)
+    {
+      const GdkEventButton *bevent = (const GdkEventButton *) event;
+      GdkDisplay *display;
+      GdkModifierType modifier;
+
+      g_return_val_if_fail (GDK_IS_WINDOW (bevent->window), FALSE);
+
+      if (bevent->button == 3 &&
+          ! (bevent->state & (GDK_BUTTON1_MASK | GDK_BUTTON2_MASK)))
+        return TRUE;
+
+      display = gdk_window_get_display (bevent->window);
+
+      modifier = gdk_keymap_get_modifier_mask (gdk_keymap_get_for_display (display),
+                                               GDK_MODIFIER_INTENT_CONTEXT_MENU);
+
+      if (modifier != 0 &&
+          bevent->button == 1 &&
+          ! (bevent->state & (GDK_BUTTON2_MASK | GDK_BUTTON3_MASK)) &&
+          (bevent->state & modifier))
+        return TRUE;
+    }
+
+  return FALSE;
+}
+
 static gboolean
 gdk_events_get_axis_distances (GdkEvent *event1,
                                GdkEvent *event2,