Drop unneeded margin inside image buttons

These margins may have been needed before GTK+ was automatically
adding the image-button style class. Nowadays, they just make the
buttons non-square.
https://bugzilla.gnome.org/show_bug.cgi?id=726500

Signed-off-by: David Zeuthen <zeuthen@gmail.com>
Version bump to 3.12.0
Revert "Updated Russian translation"

This commit introduced syntax errors in the build.

This reverts commit 29668ec44b23b91314144859d7e55c51464f6a80.
Update NEWS for release.
Post-release version-bump to 3.12.1

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