Halo visibility when no notifications option (1/2)

p1:
Unattach halo observer on window state change

p2:
hide when no notifications option

p3:
Respect mGone at halo initialization

p4:
Halo Gone: remove userhandle

p5:
Halo Gone: put back userhandle; add import for userhandle

p6: Clean some things; removed userhandle again