#782 Layer Groups phase 0: group definition/membership, Linked Layers migration, Manage dialog, show-only-group visibility (SF #222)

[Allen D Byrne]

Phase 0 of the Layer Groups feature (SF #222, feature-requests): group definition/naming/
membership, migration off the existing "Linked Layers" field, a new Manage Layer Groups dialog,
and a "show only this group" visibility action.

Scope (settled design, see SF #222 for full background):

• Data model: a Layer Group is a named, flat, user-managed list of layer memberships. Any
  layer can belong to any, all, or none of the defined groups -- no composition/derivation between
  groups (a group is never defined in terms of another group).

• Migration: the existing per-layer "Linked Layers" field (layer_t.layerLinkList, the
  LAYERS LINK <idx> "<list>" file-format line) is superseded. On load, each layer's non-empty
  link list is migrated into (or joined with) an auto-created group with a default, user-
  renameable name (e.g. "Group 1"). The old field is retired once migrated -- this ticket must
  keep reading it correctly for older files (one shipped example,
  "Ondaville Franklin and Carolina RR.xtc", uses LAYERS LINK today).

• New "Manage Layer Groups" dialog, parallel to the existing Manage Layers dialog: full
  create/rename/edit-membership/delete of groups.

• Fast "show only this group" visibility action: hides every layer not in the chosen group,
  shows every layer that is. On-screen display and printing inherit this for free since both
  already respect per-layer visibility -- no separate print/display-specific work needed in this
  phase.

• File format: this is a persisted structural change (new file-format line(s) for group
  definitions/membership, replacing LAYERS LINK), so it needs a file-format version bump
  (current PARAMVERSION is 12) and a load-time migration path for files below the new version.

Explicitly out of scope for phase 0 (tracked as later phases once this lands, per the SF #222
umbrella): native group-awareness inside individual reports, and export/import scoping. Also out
of scope: any decision about using a group in place of a single layer outside of the visibility
action above (e.g. as the current/active drawing layer) -- per SF #222's design notes, each such
UI site gets its own decision at the time it's actually investigated/implemented, not speced here.

GTK3V2MAIN only, no dual-branch port, per the established practice for new feature work given the
GTK3V2MAIN-becomes-default timeline (same pattern the Reports feature, SF #217, has used).

[Allen D Byrne]

(Phase 0 complete: git PR #181 merged into GTK3V2MAIN (27/27 CI green), Hg bug branch bug-782-layer-groups-phase0-gtk3 pushed to SF (8 commits, tip 7a653d8f3a47). Delivers the data model, file-format wiring (PARAMVERSION 12->13), Linked-Layers migration, the Manage Layer Groups dialog (dual-list Available/Included membership shuttle), and the "Show Only" visibility action. Live-tested end-to-end including against the real NYE example layout and a real user click-through session. Holding out of Hg mainline for the normal review window.