Make IBasic title / description searchable by default.

- Adds a `collective.dexteritytextindexer.utils.no_longer_searchable` function.
- Makes `title` and `description` of  p.a.dexterity's `IBasic` behavior searchable by default. Use the `no_longer_searchable` function if you don't like it ;) (Fixes #2)

/cc @lukasgraf