Multiple fields edit

Still deal with `data-editable-content`, and match that with `content-editable` new identifier value.  
Until now the value of `data-editable` was not used (hence it was thought to be a shortcut for `data-editable-template`).

* `data-editable-content` <=  `data-editable`
* `data-editable-content="identifier"` <=  `data-editable="identifier"`