Mod-defined keys and callbacks

[Unarelith]

This will act as a replacement to show_inventory and show_creative_inventory Lua functions.

Example:

mod:key {
    name = "inventory",
    verbose_name = "Inventory",
    default_key = "E",

    callback = function(client, screen_width, screen_height, gui_scale)
        -- do stuff here
    end
}