Add ability to enforce cascading selection.

Problem/Motivation

We are using the cascading selection feature of this module, which works great.
But one aspect that isn't quite right (for our particular use case), is the ability to deselect child terms, after selecting the parent term.

If a user selects a parent term, in our setup this means that all child terms are selected. So we want to enforce the selection of child terms. I.e. do not allow child terms to be selected/de-selected after a parent term has been selected.

Proposed resolution

Add a "enforce" cascading selection option when configuring the term reference tree widget field. When enabled, the child terms of a parent are checked and disabled when the parent is selected.

This will only work with "Select / deselect" option (as otherwise there would be no way to deselect child terms, and the user would be stuck).

See example of how this would work: