popupMenu: Only activate items that still have the pointer

After the introduction of implicit grabs in Clutter, a release event will now always be delivered to the actor that received the corresponding press event.

This results in surprising behavior when moving the pointer while pressed, as a button release will always activate the original item, even when the pointer moved to a different item or outside the menu altogether.

Address this by checking the hover state (that is, whether the item contains the pointer actor) before activating it.

Fixes #6606 (closed)