writerfilter: improve mc:AlternateContent support

Commit af7d4a5ebf3e6a09cd2079f241dee16aa22e0276 (n#820503: initial MCE
support in writerfilter ooxml tokenizer, 2013-07-03) introduced initial
support for this, always reading the Fallback branch. Improve this, so
an array can contain the list of features we support.

Change-Id: I2d6ba2207f6bfc62dc1e15c1a605311ede1d7eeb