This should avoid issues where this setting is changed just by opening
Gnome Control Center, as reported in
https://gitlab.gnome.org/GNOME/gnome-control-center/-/issues/918
This should also mean someone can tweak the settings as desired outside
Gnome Control Center, and not have them clobbered unless they open the
dialog in g-c-c.
Previously, there were multiple redundant lists of possible values for
this setting. This uses one struct to define not only all the possible
values, but also anything specific to the alternate characters key, so
this can be used for other modifiers.
This also changes the style of the dialog to use a GtkListBox.