Draft: Modding upgrade

The goal of this branch is to add new modding features to SRB2 that should be very very useful along with some other misc changes.

Here is a full list of what the goals are and what's done

• Support for changing the yellow menu highlight in MAINCFG (functionality ported from kart)
• Opening system menus via a Lua function for custom main menus and whatnot
• Support for changing the window title to match the mod's name via MAINCFG
• Mod options menu for lua
• Addons menu overhaul supporting mod metadata for name, author, version, and icon
• Prevent SRB2 from loading some files it really shouldn't try to load
• Try and fix addfolder crashing on Mac OS/Linux when including textures

This is all I have right now. More may be added in the future.