It's not a good idea to have STRING_LEN characters in a SwTxtNode
because then there is no valid SwPosition at the end of the paragraph.
Also it turns out that LO 3.6 and 4.0 do rather stupid things with a
full SwTxtNode. So enforce a limit, at first in the usual places that
are used during file import, SwTxtNode::InsertText() and
SwCntntNode::CanJoinPrev()/CanJoinNext().
Change-Id: Icb0f44acd20aa81635d42b84d4ae0f9b693a661c