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/qmljstools/qmljsfunctionfilter.cpp

@@ -5,8 +5,6 @@
 #include "qmljslocatordata.h"
 #include "qmljstoolstr.h"
 
-#include <extensionsystem/pluginmanager.h>
-
 #include <utils/algorithm.h>
 #include <utils/async.h>
 
@@ -81,7 +79,6 @@ LocatorMatcherTasks QmlJSFunctionsFilter::matchers()
     Storage<LocatorStorage> storage;
 
     const auto onSetup = [storage, entries = m_data->entries()](Async<void> &async) {
-        async.setFutureSynchronizer(ExtensionSystem::PluginManager::futureSynchronizer());
         async.setConcurrentCallData(matches, *storage, entries);
     };