[Development] Merging of branches (stable->dev) and reverse dependencies

[Frederik Gladhorn]

Hello,

Jędrek and I tried to finally get the changes in the stable branch merged into 
dev [1]. It has been too long and we have bigger changes, for example in the 
build system, the headers-clean test has been cleaned up to be only in qtbase.

It turns out that we cannot merge qtbase without breaking the other modules.
In the end we need to switch off the reverse dependency testing (patches in 
qtbase trigger build of jsbackend, declarative etc) for this to work.

While the reverse dep testing was great in the times before 5.0, the question 
is now if it should still be active or if we move on to disabling it in order 
to make future merges work easier.

Greetings
Frederik

[1] https://codereview.qt-project.org/#change,43948