Add a slider UI widget

I'm not too happy with how I have to configure the step for each one
separately, but it's the best I can do at the moment.

internal/config/config.go

@@ -12,7 +12,8 @@ type Ordoor struct {
 	VideoPlayer []string `toml:"video_player"`
 }
 
-// Things set
+// Things set in the options hash
+// TODO: load defaults from Data/GenericData.dat if they're not set
 type Options struct {
 	PlayMovies   bool `toml:"play_movies"`
 	Animations   bool `toml:"animations"`