[Development] Qt6 source changes

[Kevin Kofler]

Olivier Goffart wrote:
> (We should probably deprecate QQueue right now and tell people to use
> std::deque)

Please no!

The API is just not comparable: QQueue uses nicer terminology (enqueue, 
dequeue, head) and is implicitly shared, and most importantly, is consistent 
with the other Qt containers.

It is only one-sided, but often that's all that's needed. But the two-sided 
use case (for which plain QList is currently the best fit, it offers all the 
operations std::deque does, with similar wording) must not be destroyed 
either (as replacing QList with the current QVector would).

        Kevin Kofler