Adding support for an "in Watchlist" overlay for skinners

[emphatic001]

This overlay would then be a small icon, perhaps at the bottom left side of a movie poster, diagonally opposite of the "new" overlay many skins use?

[damienhaynes]

I edited title to reflect what can be done to help skinners achieve this, just to re-iterate this can only work in a list layout as thumbviews requires plugin developers to manipulate the image in memory then add it the facade.

On a selected item, the skinner can then show an overlay if the watchlist property is true.

[damienhaynes]

I think it also makes sense to expose a skin property for the 10 - scale rating. This would be useful for MovingPictures since it only has a 5 scale, this would prevent rounding issues when rating is presented to the skin.