GTK2/3 ProgressBar synchronization

Since it's not possible to have the striping effect in GTK3's default
renderer, much less with animation, this patch instead reverts both to
have a matching block-style.

Post-patch:
GTK2:
![](http://i.imgur.com/sE2jxfM.png)

GTK3:
![](http://i.imgur.com/xb4VRBg.png)