season artwork support

This is of course a first draft, so please have this in mind :)


after the changes made to JSON-RCP allowing to update season details I made some changes to the script that maybe useful until its possible to have the ListItem.SeasonId in the GUI.

It can be run with the command
XBMC.RunScript(script.artwork.downloader, mode=gui, mediatype=season, dbid=$INFO[ListItem.DBID], seasonno=$INFO[ListItem.Label]

It just needs the right condition, so it can be run from the DialogVideoInfo
For example a visible condition for a button:
<visible>Substring(ListItem.Label, $LOCALIZE[20373], left) + System.HasAddon(script.artwork.downloader)</visible>

Another thing: do the addon need to download the file locally? Can't only the remote url be added to the database? Of course not for extrafanart or extrathumbs, but this could be an option.