Qt 6 build relies on qt5compat

[gladhorn]

Currently QtWebApp built with Qt 6 needs the compatibility module. Over time it would be great to get rid of the dependency. The question is how. The compatibility is needed for QTextCodec which is used by the template engine. I'm unsure whether it would be good enough to drop the codec conversion API and require UTF-8 (I think this sounds sensible) or how important it is to keep compatibility here.

[msrd0]

I barely use C++ anymore as I prefer Rust for basically all projects. Almost all Rust libraries depend on you making sure everything is UTF-8 (actually, reading non-UTF-8 files is a real pain). So as far as I'm concerned, we can drop support for non-UTF-8 templates. That being said, keeping upstream (that deleted their github repository?????) as easily mergeable as possible would be nice ...