Add modifier grouping

Problem/Motivation

It would be nice and handy to be able to group modifiers (e.g. by section/region modifiers).

Proposed resolution

Add a taxonomy and expose this field in the modifier settings form. Make sure the groups are collapsable.

Remaining tasks

- Create taxonomy
- Add exported configuration
- Update & Install hook to add the terms "Region" & "Section"
- Update the settings form to add this taxonomy field as a selectable list
- Update the content edit form to render collapsable sections based on the taxonomy (à la field-group)

User interface changes

- Extra configuration option
- A collapsable region will be available

API changes

N/A

Data model changes

Extra field.

Release notes snippet

N/A