`alternate` is now an attr of Item::Word::Generic

Verb sets some keys to defaults: wir/sie/Sie <= infinitive; er/ihr <=
ihr/er. Added appropriate tests.
Item entry window displays chapter title

- title can be edited only for a new chapter
- added dbh argument to all ->new() in t/vokab-items.t to avoid errors
  on trying to read chapter title

- corrected use of sqlite_unicode in Vokab::DB
- Vokab::Item::* comments are added to a label returned by display_all
- removed unneeded third column in entry window
Vokab::Types have custom failure messages
Added a small script to make a new, empty database

- updated documentation of bin/vokab