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.
* added DeleteSelection to editor. Doesn't work very well yet because selection is giving me random offsets into text content.
* lots of work in the various transactions.
The key listener is now mostly a mapper of key events to editor actions. Soon, this should be 100% true.
The editor builds transactions that are themselves intelligent. The editor makes very few DOM calls itself.
It relies on the transactions to do most of the actual editing.
