macOS CICD builds

[Dirk Hohndel]

Following up on earlier discussions... I jury rigged a somewhat fragile system
that so far appears to work ok, but that I'm sure will need some adjusting to
be reliable - but for now I think I have things set up that a push to master will
deliver a web hook to one of my macOS VMs which then creates a build that
is reasonably similar to what I do for releases (except it's not signed). And that
build is then uploaded to the GitHub release.

I'd appreciate if those binaries could get some more testing (I ran it on my
work Mac as well as in a different VM). Note that these new binaries are
Qt 5.13 based, so they only run on macOS 10.13 or newer. I still have a 
setup that also allows me to manually create Qt 5.11 based builds so we 
can support somewhat older versions of macOS.

/D