Rework storage and provider structures

[abdnh]

This overhauls the "backend" of the add-on that consists of the sentence cache and the sentence providers. Major changes are the following:

1. Cached sentences are stored in a single SQLite database file named "sentences.db" instead of the old JSON files.
2. A more flexible class-based provider implementation that should allow us to add new sentence sources more easily, especially ones that support multiple languages. This also will allow us to add support for enabling/disabling specific providers.
3. All providers were rewritten to use the new implementation. Some providers like Tatoeba need more work to work out of the box though.

Closes #3 Closes #4