Qml Debugger: Separate logic for locals retrieval and focusing a frame

When debugging Qml, to update the values of the locals of a frame, a "frame" command is sent to the qml debugger. Its reponse is then handled by handleFrame which implicitly focuses the frame while dealing with the locals' values.

This caused QTCREATORBUG-29329 which focused the frame when adding a watch. This behavior doesn't happen with other debuggers.

The logic to retrieve the values of locals of a frame should be distinct from the logic to focus that frame. It should be possible to do one, the other, or both.