[a11y] No border around focused listbox rows

[Jeremy Bícha]

Ubuntu 17.10 Beta
light-themes 16.10+17.10.20170930-0ubuntu1
gnome-control-center 1:3.26.0-0ubuntu3

I originally reported this to GNOME but it looks like a theming issue with Ubuntu's default theme Ambiance.

Impact
======
It is very difficult to enable Mouse Keys if you have a working keyboard but not a mouse or touchpad or similar pointing device.

Test Case
=========
Open the Settings app (gnome-control-center)
Press Shift-Tab.
Then use the arrow keys to scroll down to Universal Access.
Press Enter to load the Universal Access panel
Press the right arrow to switch the focus to the Universal Access panel.
Press the down arrow several times to scroll down the page. The page scrolls down and you can select individual elements with the Enter key. But the bug is that you can't visually tell which element is focused.

Alternate Test Case
===================
Switch your theme to Adwaita by running in a terminal:
gsettings set org.gnome.desktop.interface gtk-theme 'Adwaita'

Now follow the steps in the Test Case.
With the Adwaita theme, the listbox rows have a dotted border around them so you can visually tell which element is focused.

To change your theme back to the default, run
gsettings reset org.gnome.desktop.interface gtk-theme