ProjectExplorer: Add a base class for build aspects

... and make use of it in the QmlDebuggingAspect.
A build setting is conceptually not a boolean, but a tri-state, as we
need to support force-switching a feature on and off as well as
specifying that it is to be left at its default value.

Change-Id: I15552614c5cf4f5187c026909d233c13e3487e81
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/buildaspects.cpp

@@ -0,0 +1,30 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "buildaspects.h"
+
+namespace ProjectExplorer {
+
+} // namespace ProjectExplorer