Add OpenGL analyzers, and make analyzers configurable (right-click).

I've ported two OpenGL analyzers from Amarok 1.x. One of them shows
a whirly background, with disco lights in front (named "Disco"). The
second one shows something like bouncing balls between two walls
(named "Balls").

One of the main advantages of using OpenGL instead of QPainter is the
significantly lower CPU load, if you have anything resembling a GPU in
your computer.

I've also made the height of the widget configurable in 3 different
sizes.

NOTE: The Balls Analyzer looks somewhat broken. I'll fix that.