GduVolumeGrid: Fix theming problem.

Just use our own CSS instead of relying on
GTK_STYLE_CLASS_NOTEBOOK. This was reported in bug 723495.

https://bugzilla.gnome.org/show_bug.cgi?id=723495

Signed-off-by: David Zeuthen <zeuthen@gmail.com>
Update NEWS for release.

Signed-off-by: David Zeuthen <zeuthen@gmail.com>
Post-release version bump to 3.12.2

Signed-off-by: David Zeuthen <zeuthen@gmail.com>