revert "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 (reverted from commit 4b7a84d0e762d0c310143c8e9f19347983370d8e)