make split handle more than two words

[Craig Berry]

The current split operation in the annotation panel only allows a single word to be split into two words, but there are reasonably frequent cases where it needs to be more than two, or there is a multi-word gap indicated by <◊>. The original idea was that there would be something like a plus sign button that, when clicked, would open a new form entry for another word, and one could add as many words as needed up to some reasonable limit. This is not what we currently have but it would be nice to have.