loginScreen: reset greeter when coming back to login screen

When a user logs in to a wayland session, we keep the login screen
running on the X server with the login screen running in a deactivated mode.

This commit makes sure it get reactivated when the user comes back to
the VT (from user switching, logout or just ctrl-alt-f1).

https://bugzilla.gnome.org/show_bug.cgi?id=726989