Solutions: Long live Solutions!

Short live Tasking in Solutions!

Add src/libs/solutions/README.md with the motivation and hints.

Move TaskTree and Barrier from Utils into Tasking object lib,
the first solution in Solutions project.

Tasking: Some more work is still required for adapting auto and
manual tests. Currently they use Async task, which stayed in Utils.
For Qt purposed we most probably need to have a clone of
Async task inside the Tasking namespace that is more Qt-like
(no Utils::FutureSynchronizer, no priority field,
global QThreadPool instead of a custom one for Creator).

Change-Id: I5d10a2d68170ffa467d8c299be5995b9aa4f8f77
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/remotelinux/rsyncdeploystep.cpp

@@ -18,7 +18,6 @@
 #include <utils/algorithm.h>
 #include <utils/process.h>
 #include <utils/processinterface.h>
-#include <utils/tasktree.h>
 
 using namespace ProjectExplorer;
 using namespace Tasking;