QtSupport: Add a QmlDebuggingAspect

... and make use of it in the qbs build configuration.
We cannot use it for qmake yet, because the build config there still has
a custom, non-aspectified widget.

Change-Id: Iacf902a2d9384d0074b10fdc98e082fa906fb6d6
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/projectconfigurationaspects.cpp

@@ -394,6 +394,11 @@ void BaseBoolAspect::toMap(QVariantMap &data) const
     data.insert(settingsKey(), d->m_value);
 }
 
+QCheckBox *BaseBoolAspect::checkBox() const
+{
+    return d->m_checkBox;
+}
+
 bool BaseBoolAspect::defaultValue() const
 {
     return d->m_defaultValue;