Toggle SparkMenuButton opened state without delay.

review @devoncarew 

This removes noticeable delay / sluggishness after click to open / close menus.
I have tested it manually in CDE.
1. Let's keep the naming consistent here, meaning `mouseDownHandler`. Event handlers are non-private throughout the widgets because when they are attached from the HTML using `{{}}`, they must be public: otherwise, they break mysteriously. So I'd prefer to keep all of them public to save us potential debugging headache in future.
1. Could you add a comment for the on-mousedown and on-click handler, explaining how they interact and why?