switcherPopup: Lookup index of items when hovering or clicking

switcherPopup: Lookup index of items when hovering or clicking

Right now the index that gets selected on click and motion events is set
when connecting the event inside the addItem function. If items are
added or removed (for example when an application is closed by pressing
"q"), this index isn't valid anymore and has to be updated.

To fix this, use the items themselves instead of the index as arguments
for the event handlers and lookup the index of the item inside the event
handler.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/167