Draft: Metadata for WADs/PK3s

This is initial work for metadata of mods. Anyone is free to continue.

This branch introduces:

1. Internal basic metadata for each PK3/WAD
2. Lua API integration (reading only) + metadata library (get, list, self)
3. Basic, currently unused game compatibility check.

This branch does not have:

1. Image/Text caching for file search
2. JSON parser (expected use with JSON once added)
3. All the UI sugar like mod menu etc.

Metadata (internally addondata_t to avoid any confusion with different kind of metadata...)

Name	Type	Accessibility
id	string	Read only
gameversion	INT32	Read only
gamesubversion	INT32	Read only
version	UINT16	Read only
displayname	string	Read only
versionstring	string	Read only
description	string	Read only
releasedate	string	Read only
type	string	Read only
links	string	Read only
authors	string	Read only
credits	string	Read only
note	string	Read only

Function	Return value(s)	Description
metadata.list()	table<addondata_t>	Returns table with every addon/file's existing metadata
metadata.get(string id)	addondata_t/nil	Returns addon/file's existing metadata based on id otherwise nil
metadata.self()	addondata_t/nil	Returns caller addon's own metadata, if invalid otherwise nil

ADDONDATA.lua