Added experimental exposure compensation and ISO control

Due to somewhat strange QtMultimedia camera API (it doesn't allow
to report whether the Auto ISO setting is currently used by backend)
the auto ISO value is returned as Invalid QVariant and internally
transformed to -1 (int) by Qt itself.

Note that QML CameraExposure item in Qt 5.3.1 or earlier returns
uninitialized integer as the 'manualIso' property value, when you
try to read it while not changing it manually. Use 'iso' property
instead.

Reporting list of ISO sensitivities and range of exposure compensation
values supported by camera is currently not implemented (neither the
exposure compensation range is accessible via Qt's QCameraExposure and
CameraExposure classes).
Removed annoying debug output on start