Now all completion models are of a single type called CompletionModel.
This model combines one or more categories. A category can either be a
ListCategory or a SqlCategory.
This simplifies the API, and will allow the use of models that combine simple
list-based and sql sources. This is important for two reasons:
- Adding searchengines to url completion
- Using an on-disk sqlite database for history, while keeping bookmarks and
quickmars as text files.
