Build Linux, MacOS and Windows platform in Github Actions CI

[DeinAlptraum]

This is a first attempt at a Github Actions CI. It currently successfully builds for Windows (32 btit), MacOS (Intel) and Linux (64bit).

There's still a few problems and open questions though:

• the Windows build and the portable upload runs for me, but the produced installer does not. Despite following the exact same steps for the installer as the current CI, it immediately crashes with the following error: I do not know what causes this error (no, "run as admin" doesn't fix this) and googling it did not really help either. Since I have no experience with NSIS, maybe someone here knows?
• Trying to do the Linux build via the Docker image (like the AppVeyor CI does) just leads to a build failure with no error message somewhere in tasks.sh, see here. I have zero experience with Docker and wasn't really able to follow what it does. I tried implementing the build, including the AppImage generation, manually as you see. This generally passes the build, but for some reason the generated AppImage crashes with the error "Could not load any language file". Apparently it is looking for the language files on the host filesystem, instead of within the AppImage. I don't know why this happens or how to fix it, and wasn't able to find any answer online either.
• I see that the AppVeyor CI also builds for Linux with 32bit. Is this a relict or do we actually want 32bit builds? I've only implemented the 64bit build so far

TL;DR this is very much a WIP except that I have no idea how to proceed with the problems I mentioned. I guess in the current state this cannot replace the existing CI, but I thought I'd still leave this here in case someone wants to pick this up or knows how to fix these problems. Not sure if GH Actions are enabled on this repo, so here's a link to how a build looks like currently on my fork.