* (#1874) Disable update checking for unsupported platforms
As described #1874, only Windows, macOS and GNU/Linux are officially
supplied with builds. Thus checking for updates is unnecessary if we
are e.g. on FreeBSD, OpenBSD, illumos distros and possibly other
Operating Systems.
This fixes#1874 by ifdef-ing the 3 officially supported platforms
when checking for updates. Otherwise a debug warning will be emitted
and in the settings the checkbox for beta-updates is hidden and
replaced by a message, explaining the reason for why there are no beta
updates.
* Update CHANGELOG
In accordance with #1874
* (#1874) Move platform check into checkForUpdates
As discussed in #1914
* (#1874) Move check for supported OS to a seperate function.
As requested in #1914
The `CHATTERINO_NIGHTLY_VERSION_STRING` defines differ on Windows and
MacOS/Linux. (See `appveyor.yml` and `travis.yml`, respectively.)
Therefore, the Windows version string looked really messed up but it
should be fixed now.
