Fix Qt5.15 deprecation warnings

[ra3xdh]

The Qt5 port #72 build and runs on Qt5.15 without major problems. But there exists a lot of deprecation warnings coming from different places in code. This will not stop the preparation of the upcoming release, but need to be fixed in the near future. The Linux distributions with Qt < 5.15 are not affected by this issue.

[ra3xdh]

I have started to fix the deprecation warnings. Now only about 10 warnings are remaining. It was required to rewrite some parts of the affected code. The current status could be seen at this branch: https://github.com/ra3xdh/qucs_s/tree/fix_5.15_warnings I will merge it in the master after everything will be finished.

[ra3xdh]

The warnings related to Qt5.15 are now fixed. The changes are merged into the current branch. The Qucs-S sources could now be compiled on all Qt version from 5.9 to 5.15.