Editor update: inform user about errors that occured during document transaction

Functional changes:

> editor: inform user about errors that occured during document transaction

Internal changes:

> smartxml: Document.transaction success callback
> editor: dialog - allow for hiding submit/cancel buttons
> wlxml: allow extending WLXMLDocument
> editor: canvas - initiate commands transactions via Document.transaction
> canvas: no need to handle nodeTagChange event anymore
> smartxml: removing special implementation for changing tag name
> Merge in canvas refactorizations
> smartxml: transaction calls error handler instead of throwing error
> smartxml: Automatically rollback transaction on error thrown
> smartxml: Rollback support for transactions
> smartxml: cleanup