QmlCppDebugger: Decouple states of engines

The main engine now follows mostly the state of the gdb engine,
only if a breakpoint is hit/while stepping the state of the qml
engine is used. This allows us to hit C++ breakpoints at any time
(also when the qml engine hasn't been able to connect yet), and
also fixes the invalid transition warnings.

Change-Id: If67a56fd28b098952be2606d0a46e04c27835f66
Reviewed-on: http://codereview.qt-project.org/5897


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>