Make context menu appear when the Menu key is pressed on the keyboard.

This fixes issue #3323.

A couple notes:
* When I tested this on my machine, the Keys.Apps key was used when I pressed the Menu key on my keyboard. I assume this will work for other users, since going by these docs (http://msdn.microsoft.com/en-us/library/system.windows.forms.keys%28v=vs.110%29.aspx), Keys.Menu is actually referring to the ALT key.
* I moved most of the common logic into a TreeViewShowContextMenu() method, which I think should work. Let me know if this or anything else needs to be changed.