TimelineView builds the event lookup once over the whole timeline and threads
it (plus canWrite) to every YearBand, so a curated event's letters cluster
under it inline. The /zeitstrahl meta-line drops its 'Gruppierung: Datum'
segment (toggle-free view, REQ-011); the now-unused timeline_grouping_date
key is removed from de/en/es and the messages parity guard, which now asserts
the new show-more/less keys.
Refs #850
8d37ee4ffb