Fix LT-14517 "Subsense number appears wrong place"

Change Subsense fwlayout parts to include cssNumber="xsensenumber"
attribute. This affects the stage 1 XHTML export to output subsense
numbers like sense numbers (ie. as ItemNumber, rather than LiteralString
nodes).

Change-Id: I4792cd6c68ba5a96f56f406c0860267300cf736b