[Development] HEADS UP: Qt 5.7.0 branching ongoing

[Oswald Buddenhagen]

The 5.7.0 branch is now available. Please start using it for changes
targeting the Qt 5.7.0 release. 

We will merge the 5.7 branch to 5.7.0 a last time on May 19th, so
there should be enough time to finalize ongoing changes in 5.7, and
start using 5.7.0 for new changes.

Changes done on 5.7.0 are forward-merged to 5.7 and dev - as usual.
Don't cherry-pick in either direction if at all avoidable - as usual.