The simplest way to deal with the problem that wrong-lists are on the
SwTextNode is to just take them as-is and iterate over them in a merged
SwTextFrame.  This may give results that users may disapprove of, but
i'm not sure how likely it is in practice to e.g. start a delete redline
in the middle of a word and end it in another paragraph.

Perhaps it would be better to move the SwWrongLists to the SwTextFrame,
but that is probably more work.

Also add missing SwTextNode function overloads for "const", so we don't
need mutable SwTextNodes.

Change-Id: I9742e3793abe090cf58ad3f27b51e89be86a1964