Turning Slow-keys on from keyboard have no effect on a11y panel and gsettings

[Jiro Matsuzawa]

Problem
=======
The Slow-keys function get enabled but it is shown as disabled in the Universal Access panel of System Settings when you turn it on from the keyboard by pressing and holding the Shift key for eight or more seconds.

Turning Slow-keys on/off from the keyboard has no effect on gsettings. It has the effect on only XKB settings. After turning Slow-keys on from the keyboard, I got the following setting values:
$ gsettings get org.gnome.desktop.a11y.keyboard slowkeys-enable
false
$ xkbset q | grep Slow-Keys
Slow-Keys = On

And I got no notification when I turned it on/off.

See also the following report:
https://bugzilla.redhat.com/show_bug.cgi?id=816764

Expected result
===============
When you turn the Slow-keys function on from the keyboard, it should be shown as enabled in the Universal Access panel.

Steps to reproduce
==================
1. Click the icon at the very right of the menu bar and select "System Settings".
2. Open "Universal Access" and select the "Typing" tab.
3. Select "Turn on accessibility features from the keyboard".
4. Press and hold the Shift key for eight or more seconds.