Add order to chapters

[benedeki]

Background

Release notes are added to chapters based on their label. And the order in the map matters (does it?) - first hit includes the release note.
The priority order doesn't have to be the same as the desired display label, though.

Feature

Add (optional) ordering property to group definition.
To consider:
Include service chapters in the ordering, too.

[miroslavpojer]

Fact & information cleanup

Release notes are added to chapters based on their label.
Answer: No. Release notes are added to chapter rows, and this process is not controlled by labels. The labels are used for placing user-defined labels to user defined and named chapters.

The order is not controllable for service chapters now.
Custom chapters are ordered as they are defined in the chapters json input.