WebUI: Implement server-side filtering.

Now you can use ```json/torrents``` request with ```filter``` parameter to retrieve filtered torrents.
Example: ```http://localhost:8080/json/torrents?filter=active```

Server-side sorting/limiting coming soon!
I just implemented Server-side sorting, but WebUI don't support it still.
What is the point of this class? Why not using simple enum? Probably with should share it with TorrentModelItem::State?