Should hopefully fix the problem, which is essentially that all
SwPageNumberFields share a single SwPageNumberFieldType, which is only
updated properly in SwTxtFormatter::NewFldPortion, hence all expansions
of a SwPageNumberField other than that return wrong values.
Does not fix the problem for fields in headers though, which appears
much harder...

Change-Id: Iecf363c8acbc2aaa418cc0c980e0b62c8e0e7d14