Add support for SVT Plays categories

Introduces a new top menu item which links to the categories that
SVTPlay manages.
Clean out unused preference functionality

This code was leftover from old plugin versions. Not used anymore.
Improve the precaching

The precaching of show information used to be broked and too agressive
in it's fetching. This new method saves the timestamp of when the show
info was last update and only updates when 30 days has passed. This
ensures that it's not done too often.

The reason for saveing the timestamp instead of using the HTTP caching
is that we need to be able to get fresh pages from the same urls that
give the show information so storing them in the cache for fast access
to show information is not an option.
Locate and store thumbnail for shows

Dig through the category sites to find the thumbnail url for every show
and store it with the rest of the show info.