Ambiance: support zebra striping for treeviews
This is a rework of Andrea's patch (committed in r220:
http://bazaar.launchpad.net/~ubuntu-art-pkg/ubuntu-themes/trunk/revision/220)
that doesn't introduce a visual glitch on progress bars
inside treeviews (because of this visual glitch, the
r220 patch was reverted in r234:
http://bazaar.launchpad.net/~ubuntu-art-pkg/ubuntu-themes/trunk/revision/234
and bug LP#945430 had to be reopened), and therefore
allows us to have a theme that implements zebra
striping treeviews when applications choose to use
them via `self.treeview.set_rules_hint(True)` API.
. Fixes: https://bugs.launchpad.net/bugs/945430.
Approved by PS Jenkins bot, Andrea Cimitan.