AsyncTask: Centralize setting global future synchronizer

Move the global FutureSynchronizer to Utils and use it
by default for AsyncTask (if that is in the main thread).
This way setting a synchronizer is less prone to be forgotten.
Individual uses can still override this.

Change-Id: I1a12bf4d7f4cb5be99668bf0a4797108c2fe6448
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/python/pythonsettings.cpp

@@ -751,11 +751,9 @@ PythonSettings::PythonSettings()
     initFromSettings(Core::ICore::settings());
 
     const auto onRegistrySetup = [](Async<QList<Interpreter>> &task) {
-        task.setFutureSynchronizer(ExtensionSystem::PluginManager::futureSynchronizer());
         task.setConcurrentCallData(pythonsFromRegistry);
     };
     const auto onPathSetup = [](Async<QList<Interpreter>> &task) {
-        task.setFutureSynchronizer(ExtensionSystem::PluginManager::futureSynchronizer());
         task.setConcurrentCallData(pythonsFromPath);
     };
     const auto onTaskDone = [](const Async<QList<Interpreter>> &task) {