Allow Editable to access relational fields

Rather than directly accessing the model's attribute (via
$this->model->{$this->attribute}), Editable will now follow the relation
chain of the model, using dot format (model.relation.attribute).
Also, $this->value will now take precedence over $this->attribute, as in
DataColumn::renderDataCellContent.
In the case of a relational attribute, the dots will also be escaped to
allow jQuery to find the element.