toolbar gradients

[Thomas Lübking]

The last commits include support for a toolbar background gradient. it's by 
default off.

In order to not introduce visible features that will be stripped or removed, 
you can change the gradient by mmb clicking the toolbar (the toolbar only, i 
think the rmb menu eats everything but leftclicks on children?!)

I also changed the layouting code for the progressbar (the boxlayout control 
is to heuristic)
The slider length currently depends on whether you use the moodbar.
Is there a signal when the AmarokConfig changes? (currently you need to 
enforce a relayout after changing the setting :-(

Cheers