[AirTunes] - fix coverart

The coverart which is streamed from airtunes clients to XBMC had issues a long time. Finally i found some time to fix it.

1. When the stream started - no coverart was shown. The first commit addresses this bug (basically we got the coverart before our player was in the playing state - so we had no item to at it too - fixed by delaying the coverart assignment to the OnPlay event)

2. When changing tracks on the airtunes client the coverart was not refreshed in XBMC. The last 2 commits fix this. (1. the texture was cached in the unused texture list and since the filename doesn't change but only the content - we always got the same texture, 2. in guiwindowhome the message for refreshing thumbs wasn't send to the base class).

Thx jmarshallnz for helping to find the proper code changes.