altTab: Check for reentrancy in override function of _onItemMotion()

altTab: Check for reentrancy in override function of _onItemMotion()

Just like in the parent _onItemMotion() function, we should check for
reentrancy in our override.

Because the hover timeout will prevent a new selection from happening
for some time, in addition to checking for this._highlighted reentrancy,
we also need to track the item that's being hovered during the timeout.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2098>