skins2: implement a ArtManager singleton

skins2: implement a ArtManager singleton

This singleton is based on the same pattern as VlcManager or VoutManager.
It is intended to load and cache art for the whole skin.
Caching makes sense because 1/ several controls within a skin can display
the current art, and 2/ art is likely to be reused by successive inputs
(music album, repeat). Yet, caching is limited to two art for memory reason.