DOCX import: never call lcl_start/endElement if handling mce elements

The problem was that after handling the mce elements
(AlternativeContent, Choice, Fallback) we still invoked the lcl version
of these methods and this resulted in duplicated properties in the
dmapper, finally resulting in incorrect position of the textbox.

Change-Id: Ib581162cba703fc1e2e44c1a9e63358891d6f78c