QmlProject: Add mainFile property

The mainFile property of QmlProject is the default file to run. People
have still the opportunity to override this in their run settings,
though.

The wizard generated code was updated accordingly. Note that this makes
projects generated by the wizard incompatible with QtCreator 2.1!

Task-number: QTCREATORBUG-3249

src/plugins/qmlprojectmanager/qmlproject.cpp

@@ -158,6 +158,13 @@ QStringList QmlProject::files() const
     return files;
 }
 
+QString QmlProject::mainFile() const
+{
+    if (m_projectItem)
+        return m_projectItem.data()->mainFile();
+    return QString();
+}
+
 bool QmlProject::validProjectFile() const
 {
     return !m_projectItem.isNull();