Python: Move plugin class definition to .cpp

The plugin classes partially changed there purpose, they are no more
meant to provide internal utility functionality.

Change-Id: I9b9200995eaa95dcd924c94dcedb28e6d5db0be9
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/python/pythonsettings.cpp

@@ -642,7 +642,7 @@ static void disableOutdatedPyls()
         disableOutdatedPylsNow();
     } else {
         QObject::connect(PluginManager::instance(), &PluginManager::initializationDone,
-                         PythonPlugin::instance(), &disableOutdatedPylsNow);
+                         pluginInstance(), &disableOutdatedPylsNow);
     }
 }