[Development] [CI] reverse dependency testing

[Rohan McGovern]

A while back, it was requested that the Qt Project CI should reject
incoming changes to QtBase if they would cause compile or autotest
failures in QtDeclarative.

That is now implemented, which should reduce the risk of accidental
source or behavior incompatible changes in qtbase.

However, if you need to _deliberately_ do some changes in qtbase which
would break qtdeclarative, it's necessary to do some additional steps.
These are documented, along with a few other bits of information, on the
wiki at http://wiki.qt-project.org/CI:_Revdep .

The reverse dependency testing is not yet set to enforcing, so it won't
block changes.  Assuming no objections, we'll probably set it enforcing
on this Thursday, 3rd May (if it is passing at that time).

Similar reverse dependency testing can be added for other modules on
request.