Don't kill the QProcess instance which is running in other thread

The main thread doesn't kill processes directly, but
sets just the cancel flag.

Instead of waiting infinitely long for a process to finish
we poll periodically (every 100ms) for the canceled flag
inside the thread that started the process.

Amends: a30aa4421a

Task-number: QTCREATORBUG-26333
Change-Id: I599d5c6cd69381cadc4a01c65f6a79f9a9bbd3fb
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -674,8 +674,9 @@ void QmakeBuildSystem::asyncUpdate()
     connect(watcher, &QFutureWatcher<void>::canceled, this, [this, watcher] {
         if (!m_qmakeGlobals)
             return;
-        watcher->disconnect();
         m_qmakeGlobals->killProcesses();
+        watcher->disconnect();
+        watcher->deleteLater();
     });
     connect(watcher, &QFutureWatcher<void>::finished, this, [watcher] {
         watcher->disconnect();