Check that application to be profiled has the debug settings

The profiler relies on the user having enabled qml debugging in the application, although this is not strictly necessary. The jsdebuggeragent and the qmlobserver are not necessary (and they even slow down the app if enabled), but qdeclarativedebughelper is needed, since that is the one that sends the profiling information.
This can be done by defining QMLJSDEBUGGER and NO_QMLOBSERVER in the app's project file, but has to be done by the user manually.

The course of action here is: factor out the connection code used in the QmlEngine for debugging, for it to be used too by the profiler with the advantage that it already detects if debugging is enabled. Then, show some error message when it's not.