Debugger: Avoid crashes when no C++-engine is available.

Check for valid runcontrols in various places and initialize
m_engine correcly. In the case of QML-Debugging, handle
NoEngineType.

Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>
Task-number: QTCREATORBUG-2086