some fixes for skins containing the cover art widget

Supported skins:
* Shade
* LateNight
* Deere

To show/hide the cover art widget use [ctrl+5] or [View->show cover art]
Ohh yeah, but the problem is that 16x16 is actually too small (nonsense) and I guess that nobody would want to keep it open just to show a tiny 16x16image.
Also, Mixxx will always start with 16x16 and it would force users to always resize it... so, maybe we should store the last size (in this way we could use 16x16 again, because the user would resize only once). What do you think?