Add extrafanart and extrathumbs to library.

[BigNoid]

Adds ListItem.Art(extrafanartn) and ListItem.Art(extrathumbsn) where n=number of downloads.
This will break current implementation used in skins where they show the images in the extrafanart path, but has advantage that skins can check on existence of extrafanart and wont have to use dirty hacks to show extrafanart/extrathumbs.

[rplotkin]

Since this is a breaking change, and since it's holding things up on Nox5 extrafanart, would you consider branching instead?

[rmrector]

I like this idea. I expanded on it so that it's not a breaking change along with some other doohickies in pull request #87.

How were you considering using them in skins? I've got a plugin that can rig images together into a list for a 'multiimage' control, but firing up the Python interpreter causes an ever-so-small delay when switching to a new list (a different TV show / movie), which is probably more noticeable on a lower powered device, though not as bad as a network folder can be now. It might be cool if 'multiimage' supported static content like lists with the element. I imagine it would be possible to rig up individual images to fade/transition, but I don't really know how to go about that. All of these are more work than the simple $INFO[ListItem.Path,,extrafanart/] skinners use now, though, and I'm not sure how to avoid that.

I'm also considering naming fanart as 'fanart#'; the 'extra' seems redundant for this. 'fanart' will stick around and should still be the choice for best/first/only fanart.

'extrathumbs' seem to just be smaller sized fanart from tmdb, though I could have swore the last time I used them them (a long time ago) they were video thumbnails. I'm not sure how useful resized fanart is, so I'll just leave it as is.

The benefits of this change will be to avoid delays in filesystem lookup, which always happens at least once when the 'multiimage' path is on the filesystem, to get a list of the images in it. If the images have been cached (viewed in Kodi), they will still be available to skins/add-ons even if the media is no longer available (unavailable network filesystem, removable media). For skins it also makes adding the single fanart to a multi fanart display fairly straightforward, while on the add-on side it's easier to ensure that the single fanart isn't duplicated in the extra fanart, depending on add-on settings.