(#1874) Disable updates on non-Windows, Linux or macOS platforms (#1914)

* (#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

src/common/Version.cpp

@@ -32,6 +32,12 @@ Version::Version()
     }
 
     this->fullVersion_ += this->version_;
+
+#if defined(Q_OS_WIN) || defined(Q_OS_LINUX) || defined(Q_OS_MACOS)
+    this->isSupportedOS_ = true;
+#else
+    this->isSupportedOS_ = false;
+#endif
 }
 
 const Version &Version::instance()
@@ -60,4 +66,9 @@ const QString &Version::dateOfBuild() const
     return this->dateOfBuild_;
 }
 
+const bool &Version::isSupportedOS() const
+{
+    return this->isSupportedOS_;
+}
+
 }  // namespace chatterino