External editors: Fall back to path if not found in Qt version.

External editors: Fall back to path if not found in Qt version.

Use Designer/Linguist executables from path if (embedded)
Qt version does not ship them.

Task-number: QTCREATORBUG-4826
Change-Id: Ide2fe0e59a8ea8f93a9193e0f97877631ce68470
Reviewed-on: http://codereview.qt.nokia.com/502
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>