Modernize code
This is a fairly big MR trying to modernize code a bit. I tried to split it into logical commits for easier reviewing.
First of all it fixes most of the build warnings. Well, at least the easy ones. There is still a warning about QNetworkConfigurationManager being obsolete, so we will have to port away from it. And I don't really see any obvious replacement. Any ideas? Another warning that I have left is obsolete QXmlSimpleReader/QXmlInputSource. Other warnings should be fixed.
I bumped C++ version requirements to C++14, so that I can use qOverload<*>(). Qt 5.14 is now needed as well.
Can you check commit "Port away from deprecated QModelIndex::child." slightly more carefully. I think I got it right, but second pair of eyes would be appreciated.
KRun is replaced with new KIO Jobs from KF 5.71 that dfaure talked about in Akademy 2020 (old KRun jobs are also marked as obsolete). This is also worth reviewing more carefully.
I've enabled QT_NO_KEYWORDS which will be default in Qt6.
Replaced almost all of the old string based signals/slots with the new functor signals and slots. (There are a few left but can't port them because LibKTorrent and Syndication don't provide overloaded functions with functors).