Qt5.15

LCG dev4 has Qt 5.15.x which deprecates quite a few things, see https://doc.qt.io/qt-5/sourcebreaks.html

A few Qt5 based packages are responsible for basically doubling the number of warnings (mostly deprecation warnings).

This MR fixes all of the warnings in dev4 except one (don't want to touch that since not sure what to do).

Unfortunately the last commit (re. SkipEmptyParts flag) is not backward compatible with 5.12.x so we can't merge this for the normal nightlies.

Instead of playing if #ifdefs etc. I would suggest we keep this MR on hold until we really have a new LCG_102 release and need it.

LGC dev4 nightlies can be setup with

cm_setup dev4