Qt5.15

LCG dev4 has switched to Qt 5.15 which deprecates many things: https://doc.qt.io/qt-5/sourcebreaks.html

So a few Qt5 based packages basically doubled the warnings in those builds (although everything still compiles).

This MR fixes a few of the issues. It still leaves one deprecation warning about the use QString::sprintf().

The last commit in this branch unfortunately is not compatible with Qt 5.12 that we use.

Instead of adding #ifdefs I would suggest we only merge this when an LCG release with Qt 5.12 actually appears and we switch to it.

The LCG dev4 nightlies can be used with

cm_setup dev4