Use CDATA to encode switch-input-source default binding

Currently entities are being used, but CDATA is more readable and more
consistent with the rest of the file.

https://bugzilla.gnome.org/show_bug.cgi?id=732293
Add default binding for {switch,cycle}.*-backward bindings

Currently mutter/gnome-shell make sure that the binding for a -backward
action is the same binding as for the non-backward action with the
<Shift> modifier added.
This logic is being moved to gnome-control-center for more flexibility
(the keybinding panel will suggest to set the -backward binding to
<Shift>+non-backward-binding).
Since mutter/gnome-shell will no longer be automatically creating these
<Shift> backward keybindings, they need to be added as default values
to the -backward gsettings keys.

https://bugzilla.gnome.org/show_bug.cgi?id=732293