(#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/widgets/settingspages/GeneralPage.cpp

@@ -5,6 +5,7 @@
 #include <QScrollArea>
 
 #include "Application.hpp"
+#include "common/Version.hpp"
 #include "singletons/Fonts.hpp"
 #include "singletons/Paths.hpp"
 #include "singletons/Theme.hpp"
@@ -540,10 +541,20 @@ void GeneralPage::initLayout(SettingsLayout &layout)
     layout.addCheckbox("Title", s.headerStreamTitle);
 
     layout.addTitle("Beta");
-    layout.addDescription(
-        "You can receive updates earlier by ticking the box below. Report "
-        "issues <a href='https://chatterino.com/link/issues'>here</a>.");
-    layout.addCheckbox("Receive beta updates", s.betaUpdates);
+    if (Version::instance().isSupportedOS())
+    {
+        layout.addDescription(
+            "You can receive updates earlier by ticking the box below. Report "
+            "issues <a href='https://chatterino.com/link/issues'>here</a>.");
+        layout.addCheckbox("Receive beta updates", s.betaUpdates);
+    }
+    else
+    {
+        layout.addDescription(
+            "Your operating system is not officially supplied with builds. For "
+            "updates, please rebuild chatterino from sources. Report "
+            "issues <a href='https://chatterino.com/link/issues'>here</a>.");
+    }
 
 #ifdef Q_OS_WIN
     layout.addTitle("Browser Integration");