Allow use of $NUMBER[] to specify a number in XML tags

Currently numbers in `<label>` et. al. are interpreted as localized string lookups.  This means if the skinner actually wants a number in a label they have to do complicated stuff.

This is a workaround until we remove the number interpretation and force use of $LOCALIZE[] post-Gotham.  With refactored code it's a 2-liner, so not exactly a maintenance burden.