Simplify adding new regions

Problem/Motivation

Either selecting an existing - now disabled region or typing the name for a new region can be confusing.

Proposed resolution

Only allow to type the name for a new region. This is somewhat the opposite of #1722136: No way to add (back) existing regions.

Remaining tasks

A review of the patch.

User interface changes

No select box any more at the "Add region to top/bottom" modal dialog form.

API changes

None.

Data model changes

None.