Add Universal Access status indicator

Introduce the Universal Access status indicator as designed, modeled
after the similar UI provided by g-s-d. This indicator allows the user
to change rapidly the keyboard and mouse behaviour (sticky keys, slow
keys, bounce keys, mouse keys), as well as the enabled ATs (magnifier,
screen reader, screen keyboard) and the HighContrast Gtk theme.

https://bugzilla.gnome.org/show_bug.cgi?id=624916