QCameraDevice: Expose photoResolutions property

Expose photoResolutions as a property of QCameraDevice. Also update
documentation to match use case and implementation.

QML regression tests for property added to a new tst_cameradevice test,
and one CameraDevice test moved there from tst_cameraformat.

[ChangeLog] Expose QCameraDevice photoResolutions as a property.

Change-Id: I5adf12c0d122471b457cb258e6be0f1453e7a927
Reviewed-by: Nils Petter Skålerud <nils.petter.skalerud@qt.io>