Implements #8263.
Qt4.x has been EOL since 2015, and at least Gentoo has, or is going to drop support for it. I wouldn’t be surprised if other Linux distributions follow.
This removes Qt4 detection from the build system, as well as removes all Qt4 fallbacks from the code. Turns out there’s more than I expected: this is going to make maintenance of the GUI code, as well as adding new features significantly easier.
(I know there’s still some references left to qt4 in RPM and Debian build script, but I don’t have the knowledge how to fix them)