QImageCapture: Expose resolution property

Expose resolution as a property of QImageCapture. Also update
documentation to match use case and implementation.

QML regression tests for property added to a new tst_imagecapture test.

[ChangeLog] Expose QImageCapture resolution as a property.

Fixes: QTBUG-113187
Change-Id: I78fd6be2f7949071a608efbaf791b007ff7f680b
Reviewed-by: Nils Petter Skålerud <nils.petter.skalerud@qt.io>