Introduce Github Actions workflow for Cmake Ninja build

Has minimal configuration in order to achieve fastest build times.
CCache is on, and PCH is off as it significantly reduces the Ccache hit ratio.
This Github Workflow has been measured to take around
+ 30min without any Ccache hits and 8min  for a rebuild, on Linux
+ 20min without any Ccache hits and 4min  for a rebuild, on macOS
+ 1h    without any Ccache hits and 20min for a rebuild, on Windows

The goal is to use such Cloud building and testing systems as "smoke runs",
providing quick feedback, much sooner than the canonical approval from
Coin. Additionally having many independent systems helps verify "flakiness"
issues. Another big advantage of such systems is their hackability, allowing
someone to tweak the build parameters by just editing a couple of YAML lines.

Change-Id: I7eca22f513516b9e61f333fb1fe89ab11ed2d1d1
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

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