popupMenu: Connect after to item 'activate' signal

popupMenu: Connect after to item 'activate' signal

A menu item activation might lead to a call to `itemActivated` which eventually
will close the menu which leads to a `PopupMenu.removeAll` that destroys all the
items, stopping the emission of the 'activate' signal for them.

Before commit 4258ae3e this was not happening because destroy'ing a javascript
object wasn't really disposing it and thus stopping the signal emissions.

So, ensure that `itemActivated` is called after that all the other callbacks
have been consumed, and so that the menu is closed as last thing.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1326

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/557