Add MCU build step to the QmlBuildSystem

If a qml project is marked as qtForMCU the buildstep will
run the qmlprojectexporter on the currently active qmlproject
when pressing the "Run" button. The output messages of the tool
will then show up in the "Compile Output" panel.
The build step itself can be seen when switching to "Projects"
mode under "Deployment".

Change-Id: I5ac31d5655e3b4b6137aaf541839776f144a09c4
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: Burak Hancerli <burak.hancerli@qt.io>

src/plugins/qmlprojectmanager/qmlprojectplugin.cpp

@@ -1,6 +1,7 @@
 // Copyright (C) 2016 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
 
+#include "mcubuildstep.h"
 #include "qdslandingpage.h"
 #include "qmlprojectplugin.h"
 #include "qmlproject.h"
@@ -93,6 +94,7 @@ public:
     QPointer<QMessageBox> lastMessageBox;
     QdsLandingPage *landingPage = nullptr;
     QdsLandingPageWidget *landingPageWidget = nullptr;
+    MCUBuildStepFactory mcuBuildStepFactory;
 };
 
 QmlProjectPlugin::~QmlProjectPlugin()