If the engine is running and no specific context is selected, then it's
the global QML context. The same holds if the engine is running, but
the server doesn't support contextual evaluation. If the engine is
stopped, then it's a frame in the stack trace. Otherwise we don't have
a context and clear the label.

Change-Id: I5a322098f3297895e07bcc96ee4a5f351423c206
Reviewed-by: hjk <hjk@qt.io>