writerfilter: introduce ooxml::OOXMLUniversalMeasureValue

In transitional DOCX, ST_UniversalMeasure is in practice a simple integer,
which means twips. But in case of strict, ST_UniversalMeasure is in points
in practice -- which is perfectly valid, but we didn't handle it so far.

Add a separate Value class that is used only for handling
ST_UniversalMeasure, then there we can handle the various additional
suffixes.

Change-Id: Iebb1ee859076595594d1455a1f826841dae77a0b
DOCX import: handle points in ST_TwipsMeasure and ST_SignedTwipsMeasure

This fixes page size and margins in case of strict DOCX.

Change-Id: I65894eeef191a0f4bd92d1fa69a17e820aae3a43