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/libs/utils/futuresynchronizer.cpp

@@ -3,6 +3,9 @@
 
 #include "futuresynchronizer.h"
 
+#include "qtcassert.h"
+#include "threadutils.h"
+
 /*!
   \class Utils::FutureSynchronizer
   \inmodule QtCreator
@@ -62,4 +65,18 @@ void FutureSynchronizer::flushFinishedFutures()
     m_futures = newFutures;
 }
 
+Q_GLOBAL_STATIC(FutureSynchronizer, s_futureSynchronizer);
+
+/*!
+    Returns a global FutureSynchronizer.
+    The application should cancel and wait for the tasks in this synchronizer before actually
+    unloading any libraries. This is for example done by the plugin manager in Qt Creator.
+    May only be accessed by the main thread.
+*/
+FutureSynchronizer *futureSynchronizer()
+{
+    QTC_ASSERT(isMainThread(), return nullptr);
+    return s_futureSynchronizer;
+}
+
 } // namespace Utils