editor: start using data api on text nodes for keeping references to canvas elements

editor: start using data api on text nodes for keeping references to canvas elements

GenericElement doesn't need to dispatch nodeTextChange events
anymore as TextNodes now have their own reference to canvas
elements and wlxmlListener can dispatch to them directly.

We still need to handle changes to the comments though - this should
be refactored out to some kind of non-rendering canvas element.