views: Support top menu switching with mouse and arrow keys (issue #2102)

- cefclient: Add example ViewsMenuBar implementation and show top menu
  by default (specify `--hide-top-menu` to hide it).

include/views/cef_button_delegate.h

@@ -54,6 +54,12 @@ class CefButtonDelegate : public CefViewDelegate {
   ///
   /*--cef()--*/
   virtual void OnButtonPressed(CefRefPtr<CefButton> button) =0;
+
+  ///
+  // Called when the state of |button| changes.
+  ///
+  /*--cef()--*/
+  virtual void OnButtonStateChanged(CefRefPtr<CefButton> button) {};
 };
 
 #endif  // CEF_INCLUDE_VIEWS_CEF_BUTTON_DELEGATE_H_

include/views/cef_menu_button.h

@@ -78,6 +78,13 @@ class CefMenuButton : public CefLabelButton {
   virtual void ShowMenu(CefRefPtr<CefMenuModel> menu_model,
                         const CefPoint& screen_point,
                         cef_menu_anchor_position_t anchor_position) =0;
+
+  ///
+  // Show the menu for this button. Results in a call to
+  // CefMenuButtonDelegate::OnMenuButtonPressed().
+  ///
+  /*--cef()--*/
+  virtual void TriggerMenu() =0;
 };
 
 #endif  // CEF_INCLUDE_VIEWS_CEF_MENU_BUTTON_H_

include/views/cef_window_delegate.h

@@ -113,6 +113,7 @@ class CefWindowDelegate : public CefPanelDelegate {
   /*--cef()--*/
   virtual bool OnKeyEvent(CefRefPtr<CefWindow> window,
                           const CefKeyEvent& event) { return false; }
+
 };
 
 #endif  // CEF_INCLUDE_VIEWS_CEF_WINDOW_DELEGATE_H_