made a special case for typing with BR as the selection (as after an InsertBreak.)
made DEL and BACKSPACE at the beginning and end of a text node
fixes bugs 3742 and 3756 (with permission from choff and gkostello)
and merges in subsequent transactions indiscriminately until it's told to stop.
It also gives the last transaction in its child list a chance to merge the
next transaction.
All this is in support of complex transactions that result in text insertion
being able to collapse into a single undoable event.
Also improved tracking of bogus content node used when document is empty.
which calls generic WillInsert, DidInsert to test for magic bogus
content and does the right thing.
You now can't delete magic content with user gestures, and when you
insert anything the magic kludgy is deleted first.
attribute MOZ_EDITOR_BOGUS_NODE set to TRUE. this solves the problem of when you do a SelectAll then a DEL, you don't have a cursor
or any way to set selection in the document. You do still have the problem that the nbsp is pretty narrow and the frame containing it is hard to hit
with the mouse. We really need the "set selection to frame nearest the mouse down point" operation.
the intent is for all inserts to look for this node, and if found delete it before the insert. likewise, all deletes will be no-ops if this node is found.
