QtCamera2: update and/or document adding image readers

In QtCamera2.java, we set 2 listeners: one for frame capture, and another for photo capture. Consequently, QAndroidCamera emits 2 signals: onCaptured and newVideoFrame.
It's not clear why we do so, and the reason should be documented (just a few lines of comment in the code), or the code should be simplified.

If we keep 2 listeners and 2 signals, please rename onCaptured -> photoCaptured or smth like this.