Allow the possibility to complete empty an editing hosts

Updated deleting operations to allow the possibility to completely empty an
editing host of its contents (ie: CTRL+A, DEL).
Lint recent code changes in engine.js
Enable deletion of solo <p><br></p> in editable

Calling delete on the given markup:
<editable><block><br>[]</block></editable>
should results in:
<editable>[]</editable>
Remove cursor-styles in table after editing