[Development] QTBUG-24128/24129/24130: finish up platform specific code

[lars.knoll at nokia.com]

These need to get done for 5.0. Most of this is about feature regressions
against 4.x, so the deadline end of next week doesn't apply. Nevertheless
it would be good if you can try to follow the guidelines as much as
possible:

* If you need changes in the QPA interfaces, please synchronize between
the maintainers of all backends.
* If the changes are more intrusive and might break downstream projects do
them in api-changes.

Cheers,
Lars