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

js/misc/loginManager.js

@@ -39,6 +39,7 @@ const SystemdLoginSessionIface = '<node> \
 <interface name="org.freedesktop.login1.Session"> \
 <signal name="Lock" /> \
 <signal name="Unlock" /> \
+<property name="Active" type="b" access="read" /> \
 </interface> \
 </node>';