Implement EPG genre support

[dagwieers]

Currently the EPG we offer does not include "genre" (XMLTV categories).

The problem is that from the VRT NU EPG information we do not get the category.
And IPTV Manager currently does not support more than one category.

[mediaminister]

I managed to get the genre from VRT NU Search API based on the whatsonId in the Schedule API, but this only works for episodes that are already published in the Search API: https://github.com/mediaminister/plugin.video.vrt.nu/tree/genre