[Development] QArrayDataObs/Pointer UB post-mortem

[Tim Blechmann]

> - only features shouldn't be picked back, in particular
>    - test changes should always be picked back, with a XFAIL, if necessary
>    - refactorings should always be picked back (or not done at all)
>    - bugfixes should always be picked back

a word of caution: *every* change has the risk of regressions, bugfixes 
have risks of regressions, refactorings have risks of regressions. the 
more changes that are backported to stable / lts releases, the higher 
the risk they will be released with regressions