Increase the width of the volume slider widget by a factor of 2x, as discussed with Leinir and nhnFreespirit.

Reasoning: For use with touch screens, the precision of the shorter volume slider was insufficient.
E.g. it was impossible to select 1%; instead it jumped directly from 0% to 4%.

It would have been possible to increase the length as a special customization for touch screens, but we discussed on IRC that this change might also be beneficial for Amarok proper, so I'm committing it directly to trunk.