... so use it only once.  The problem is that the second time it will
call DelFrames() on every non-first node, which then empties the first
frame's newly created MergedPara instance.

This can be reproduced by toggling Edit->Tracked Changes->Show in
tdf99766-2.odt.

Also change some function parameters on e.g. SwTextFootnote::DelFrames()
to SwRootFrame to make it obvious that these don't actually need a
particular SwTextFrame.

Change-Id: I129be5db4a30546905c24c5e8820a5f105a33109