HaikuDepot: Simple TextListener interface, not yet used.
Paragraph: Added Remove(), untested.
TextDocument: Implemented Remove(), untested.

Also some refactoring for ParagraphAt() into a separate ParagraphIndexFor(),
used by the former, since often an index is needed.
TextEditor: Resolve TODO and call Insert() and Remove()...

... which both already exist in the TextDocument API (stubbed out or
untested as they are).
Text stuff: Bugfixes in Insert and Remove methods

Still not well tested. Also some way to trigger the layouts to adopt to
the changed document paragraphs. Not yet via the new TextListener.