Switch Width/Height to mm

[whot]

IIRC, originally we added w/h to make it possible to refer to devices as "That's the 6x9 intuos 3". Since Wacom uses inches, the width/height is specified in inches.

We now have quite a few devices where we just round and hope, based on the assumption that it doesn't matter that our 6 inch device is actually 5.5 inches or whatever. We should switch the data files to use mm which have a better granularity and add an API to export those mm.