Many relatively minor changes to the PQSH class.

Leo build: 20140828175722
Added LeoQTextBrowser.getTextLength and onTextChanged.  These will conflict with delayed loading of body text, and will soon be removed.

Leo build: 20140828175948
Added doc re c.cloneFindParents and clone-find-parents command.

Leo build: 20140828180225
Work related to speeding up loading large body text.
Added much better tracing in selectHelper and its helpers.
Added keyword s=None argument to setInsertPoint.
This allows a big speedup in setSelectionRangeHelper and v.restoreCursorAndScroll:
they no longer needs to compute the length of the new text.

Leo build: 20140828180714