Allow loading of projects even with no valid Qt

This patch marks up invalid Qt versions as Desktop.

Make sure we add a the always existing invalid Qt version in the path
available in the targetselectionpage if no valid Qt versions could be found.

This way a project can be set up and loaded without a valid Qt version set up.
Trying to build this will fail of course. This is basically how we handled
this situation before we had targets, too.

Task-number: QTCREATORBUG-1066
Reviewed-by: dt

src/plugins/qt4projectmanager/qtversionmanager.cpp

@@ -1155,8 +1155,10 @@ void QtVersion::updateToolChainAndMkspec() const
     m_toolChains.clear();
     m_targetIds.clear();
 
-    if (!isValid())
+    if (!isValid()) {
+        m_targetIds.insert(Constants::DESKTOP_TARGET_ID);
         return;
+    }
 
 //    qDebug()<<"Finding mkspec for"<<qmakeCommand();