Setup 3rd/4th level characters (typographic layout)

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?

In DConf (to edit it I've installed dconf-editor) go to org.gnome.desktop.input-sources. The target property is xkb-options. If «Alternative Characters Key» was already set to R.Alt, one could find 'lv3:ralt_switch' option in the list represented by this property. To enable typographic layout on 3rd/4th level one should add 'misc:typo' option. After this xkb-options will look like: ['grp:alt_shift_toggle', 'grp_led:scroll', 'lv3:ralt_switch', 'misc:typo'] and typographic layout will become enabled as well.

Upd: Without dconf-editor it is possible to update xkb-options using gsettings like this:

$ gsettings get org.gnome.desktop.input-sources xkb-options
['grp:alt_shift_toggle', 'grp_led:scroll', 'lv3:ralt_switch']
$ gsettings set org.gnome.desktop.input-sources xkb-options \
"['grp:alt_shift_toggle', 'grp_led:scroll', 'lv3:ralt_switch', 'misc:typo']"