CheckBox, RadioButton: Expose label spacing pixel metrics
The default value upstream seems to be 6dip, which is not what Breeze really uses for layouting. Interestingly, qqc2-desktop-style has the needed infrastructure in place to query these pixel metrics from a native style, so QtQuick apps have used the wrong value in for quite some time now.
As a drive-by clean-up, let's collapse those for identical cases above.