fix: #97 by adding missing snow unit cm

This MR will add a new setting "snow unit" to allow picking mm, inch or cm as snow unit. The determination of the unit had to be reworked to do this.

For focal I could test only via clickable desktop yet. But since it is only qml code changed, that should work there too. Once this is merged, I can prepare another MR cherry picking here to add this code to main as well.