This reverts commit b3722f7063.
In some cases ardour shows context-menu on right-mouse-button
release. In this case selecting a menu-entry should happen
with the left-mouse button (or any button?!)
Using ev->button is only correct if the menu is temporary and only
visible while the button is held, button release then activates the
menu-item.
This needs further work, in some cases allowing any button (0) to work
makes sense and overall consistency needs to be improved.
Different places use different strategies for context-menus which
don't always match the button used in the event-handler.
This is a hotfix (to make TAV context menus work again with left-click)
