popupMenu: Handle key-press events on sourceActor

The behavior of opening/closing/navigating a menu from its source
actor is generic enough to not limit it to PanelMenu.Buttons, so
move the code into PopupMenu itself.

https://bugzilla.gnome.org/show_bug.cgi?id=735614
popupMenu: Use an appropriate key to move focus into the menu

The code from PanelMenu.Button assumed menus would open below their
source actor, making KEY_Down a good choice; however with the new
generic code, we should base the key used on the actual menu position.

https://bugzilla.gnome.org/show_bug.cgi?id=735614