The return value of cameraParams() should be usable to restore the camera state.

Before this PR, the following would need to be done

win = GLViewWidget(rotationMethod='quaternion')
cp = win.cameraParams()
del cp['elevation']
del cp['azimuth']
win.setCameraParams(**cp)

After this PR, the following will work

win = GLViewWidget(rotationMethod='quaternion')
cp = win.cameraParams()
win.setCameraParams(**cp)

fixes #3407

Additional fixes:

1. reset() did not reset the default in quaternion mode.

NOTES:

1. Irrespective of the rotationMode, either rotation or (elevation, azimuth) can be used to modify camera params. However, rotationMode determines which internal state gets modified.