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.
I don't like the idea of having two functions QTorrentHandle::eta() and QBtSession::getETA() with a different meaning.

If QTorrentHandle::eta() is used only in WebUI, perhaps it should stay in WebUI. Also this looks suspiciusly similar to case TR_ETA in TorrentModelItem::data(). Perhaps the code should be shared?