Feature/1194 native alt field

[i-just]

Description

This is pretty much Brian’s PR with some compatibility tweaks.

Adds support for native fields, specifically aimed at the native alt field. Native fields are set with $element->setAttributes() rather than $element->setFieldValues(), which makes this a little more involved.

If support for more native fields needs to be added in the future, as long as they are mapped to a nativeFields key on the $fieldInfo they will be saved appropriately.

Related issues

#1194
#1391