Settings API improvements

[marchc1]

Significantly removes the amount of setup to create a server-side setting.

Previously, to set up a server-side setting, you had to call several functions in three different files...

---

acf/core/globals.lua

ACF.RequireFuel          = true

acf/menu/data_callbacks.lua

RequireFuel = function(Player, _, Value)
    local Bool = tobool(Value)
    if ACF.RequireFuel == Bool then return end
    ACF.RequireFuel = Bool
    if CLIENT and not IsValid(Player) then return end
    Message("Info", "Engine fuel requirements have been " .. (Bool and "enabled." or "disabled."))
end,

acf/persisted_vars/vars_sv.lua

ACF.PersistServerData("RequireFuel", true)

---

... which can now be consolidated into a single call in acf/core/globals.lua:

ACF.DefineSetting("RequireFuel", true, "Engine fuel requirements have been %s.", ACF.BooleanDataCallback())

[marchc1]

Hold on; going to try something to make panels simpler as well

[LiddulBOFH]

Looks good, will definitely make this sort of work much easier. Another notch towards #230 as well.

[marchc1]

Going to merge to dev then