Switch to Qt6

I realize most RetroArch users (especially those on Windows) couldn't possibly care less what goes on under the hood, as long as they can run it and play their games.

The problem on Linux, however, is that we like to install software via package managers. And because RetroArch is built using Qt5, this means we have to have that installed too, alongside any dependencies that requires.

This complicates a bunch of things - for example potential conflicts between platform themes, not to mention a whole lot of disk space wasted on libraries - and additional problems in regards to software development.

Qt6 has been out for years now, and many well-maintained apps transitioned to it. Except RetroArch.

So, if RetroArch could do this too, we can finally liberate ourselves from the need to have it installed.

Thanks for considering.

The issue names no files, tests, or entry points. Start by locating RetroArch's Qt5 build configuration and usage, then assess the Qt6 migration requirements; done means the project builds and runs with Qt6 without requiring Qt5.