Fixed various deprecation warnings from Qt5 and KF5
Fixed all the warnings originating from a system with Qt-5.15.2, KF-5.85 and GCC-10.3 compiler. Please see individual commits for each kind of warning. A few of them were not straightforward and I spend quite some time researching new (and sometimes old) APIs to understand how to convert the code. Please test the parts of Krusader that were touched. Most essential parts that need attention:
- Running commands and opening files of various mime types with Enter or click.
- File context menu.
- KrViewer - viewing and editing various types of files.
Please let me know if there are any regressions or corner cases (I found none so far). Thanks!
FIXED: Calls to methods deprecated in modern Qt and KDE Frameworks libs