Qt4 Cleanup

[a17r]

Follow-up to #36.

• Qt4 is unmaintained upstream for many years.
• Support for <2.8.12 will soon be dropped from a future CMake version.

[drfiemost]

Now that Qt6 is out it would be time to merge this and move on (#40)
Is there someone still maintaining the project? @ben-xo ?

[ben-xo]

Heya. This project is looking for a new maintainer. From my point of view its purpose is to support the last fm desktop app, which is also lacking a maintainer. I'd be very happy to endorse your changes - I just have no real way to test them at the moment. If you can get the desktop client to build in the relevant environments, that would amazing. Otherwise maybe start a fork and we can consider reintegration at a later date

[a17r]

I'd say there is basically no environment left with the necessary packages to build lastfm-desktop, which is a consequence of still depending on Qt4+Qt4WebKit.

https://repology.org/project/lastfm/versions
https://repology.org/project/lastfm/versions

When this PR is merged, then for sure lastfm-desktop will be broken unless ported.

[drfiemost]

There is at least Clementine player that depends on this library and I have my own project which I'm currently porting to Qt6.
I can help testing the build and basic scrobbling API on Linux and Windows if needed.
Hope that someone could step up as maintainer.

[drfiemost]

I've added Qt6 support, along with a few fixes, it's available here
Went quite smooth, the only downside is that the functions returning QMap no longer support multiple values with the same key, for which a QMultiMap is required.
Now it needs some testing, especially on Mac OS.