Delay initialising GUI when running saved selections

Loading the GTK plugin is slow and we only need it if there are missing
selections.
Removed misleading comment
Simplified SAT API

Instead of exposing variables and literals, just use literals
everywhere. Avoids awkward (and incorrect) "type var = lit".

Don't expose var_info type or the (confusing) get_varinfo_for_lit.
Instead, just expose get_user_data_for_lit.