Rename db.t -> vokab-db.t; minor DB model changes
Various minor changes

- verb entry field uses German person names as hash keys (not s1, p2,
  etc.)
- item tests use a hash for each attribute, not a list
Test accessors for Vokab::Item::* entry attributes

- attributes are now read-only
- attributes now have isa => Gtk2::Widget
- test Vokab::Item::* attributes throw the correct exceptions on failure