Covert Art: drop shadows using QGraphicsDropShadowEffect()

CoverArtShadow.png image was replaced by one without shadows, instead
shadows are now drawn by QtGui.QGraphicsDropShadowEffect(), which permits
a clean display of all images whatever are their proportions.

Examples of rendering after this patch:
![example_shadow](https://f.cloud.github.com/assets/151042/84247/7d41da4a-6406-11e2-9ba8-66d6bd1a8c5a.png)