bug 1062136 - Event driven screenshot overlay. r=alive,etienne

Merge pull request #24149 from nullaus/v2.1

Merge pull request #23874 from nullaus/bug1044125 a=fabrice
(cherry picked from commit f1d3cd2301b604b7b8b8dcf69bd36ba638c1ab44)
Merge pull request #24404 from nullaus/bug1062136-v2.0

Merge pull request #24149 from nullaus/v2.1