When Ubuntu used GNOME, there was support for typographic characters on the third/fourth level, independent of input source (e.g. «English US» or «Russian»). As I've learned, in Ubuntu 15.04 the option for accessing alternative characters has appeared again under «Keyboard -> Shortcuts -> Typing -> Alternative Characters Key». But there are no any alternative characters at all.
How can I setup 3rd/4th level alternative characters?