GitHub Actions: Merged Qt 6 based packages with the main workflow

Now building all packages in a single workflow, which helps sharing code
using a build matrix and to upload all artifacts at once when all the
builds have succeeded.

The Qt 5 based macOS builds now use Qt 5.12, so that they now even
support macOS 10.12. The Qt 6 based builds need at least macOS 10.14.

The automatic itch.io upload hasn't been used since I'm no longer
publishing snapshots there, so it has been moved to a separate file,
potentially to be used for uploading releases to itch.io in general (but
it still needs adjustments for this).

The Qt 6 based builds now also exclude Python from the AppImage, to
avoid a crash when the Python plugin is enabled.