Github CI, MacOS and Mingw build fixes

The first commit "add github ci" is obviously for my own convenience, but the MR also adds two little source changes to fix issues with our current macos and mingw builds I encountered:

• "macos build": I found that on GitHub at least, the CI would fail with a timeout while trying to execute hdiutil detach during construction of the dmg. So I added two retries in order to make that go through. This doesn't affect manual builds and shouldn't do any harm in the gitlab ci either, because if the first hdiutil detach succeeds without timing out, then of course the retries won't be run.

• "mingw build": In the latest msys2, the version of libpcre pulled in while installing the mingw build dependencies isn't libpcre any more, it's libpcre2. This causes the build to fail in a pristine build environment since libpcre-1.dll doesn't exist in the build environment any more (and it's the wrong dll to include anyway). Therefore I adjusted the inno Makefiles so that they copy the right dll, which now is libpcre2-8-0.dll.

It goes without saying that having macos and mingw builds as well as releases fully automated on the GitHub mirror makes my life much much easier now. :) Therefore I'd really appreciate it very much if this MR could go in, so that I can keep the GH mirror in perfect sync with your gitlab in the future. Thanks!