use raw text for DocumentTextBox

The text content of a DocumentTextBox can not contain any tag markers,
so it far more efficient to use GapContent#getRawText() here.

Change-Id: Ied3f048f90cead0a251b0c47a63f54d139a1299c
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
3 files changed