Setting refactor

[mccreery]

• There are 3 useless interfaces ISaveLoad, IGetSet and IBoolean associated with settings
• get and set should be getValue and setValue to be consistent with other accessors
• draw should be renderExtras
• In no case is a variable with type Setting<T> other than Setting<?> ever used; removing the type parameter allows subclasses to use primitive value types and different signatures
• HudElement stores a dubious RootSetting, which could easily be a natural list of settings
• Global/mod settings are stored as a whole element, despite not showing up in the list; mod settings should be a list of settings just as element settings are
• Nested settings are pointless and make the config file format messy; instead, complex settings should store a JSON object instead of a value directly
• Setting categories are implemented incorrectly: categories are not used and should group and produce legends for setting screens
• Legends should be replaced by proper setting category structure; this structure need not be reflected in config files
• New Widgets remove the need for setting callbacks, though updating widgets is still necessary