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.
Merge branch 'progress-bars' of https://github.com/wrouesnel/mint-themes into wrouesnel-progress-bars

Conflicts:
	usr/share/themes/Mint-X/gtk-2.0/gtkrc