pressing enter text view should add new numbered line

[jonorthwash]

Currently pressing Enter results in either invalid CoNLL-U (and maybe CG3 and SDparse), or splits the "trailing" text into a new sentence.

Instead, it should result in an entire new valid token line (with numbering if needed, e.g. in CoNLL-U, and additional lines, as in CG3).

There should be a different keyboard shortcut for splitting into two sentences, e.g. Shift+Enter.