Add library for creating GUI menus

[Ghoulboy78]

Much like classes.scl, this will in theory allow players to create their own GUI libraries by declaring a list of functions in a map.
Unlike classes.scl, this might actually be useful, since it abstracts a lot more of the grittier process behind nice-to use functions and syntax. Also, this has more flexibility.

Fixes #315

Todo items:

• Add basic (static) buttons which can do stuff when clicked
• Add dynamic buttons (clicking the button changes the icon, edits inventory itself, etc)
• Add storage (so items u put in can be saved)
• Add dynamic storage slots (Whenever this slot is modified, a function is called)
• Add ability for programmers to insert their own code into screen callback (with access to (screen, player, action, data) variables, as well as gui_screen variable)
• Add pages made of buttons
• Give different titles to new pages (Requires closing and opening new screens)
• Allow for different sized pages (Same here)
• Add button labels
• Allow to use any gui type, not just generic rectangle ones (Need to make sure they don't break existing stuff, and we're good)
• Add more triggers to change page (basically extract page-changing into a separate function that can be called from anywhere)
• Add special abilities for all the different types of inventories (Not all types need special treatment, but I'll add them all here, just so I can tick them through, and incase someone else thinks of what to do with this)
• Anvil (Maybe detecting when the player writes? If the player is renaming, each character change updates the item in the output slot, causing a slot_update action to be sent out)
• Beacon (Effects ig, gotta read the docs Nothing, as it happens)
• Brewing stand (Nothing that I know of)
• Blast furnace (I'll redirect this to whatever the furnace does)
• Cartography table (Nothing that I know of)
• Crafting table (Selecting a crafting recipe)
• Enchanting table (All the enchantments, this is gonna be painful, I think)
• Furnace (Selecting a smelting recipe)
• Grindstone (Nothing that I know of)
• Hopper (Nothing that I know of)
• Lectern (Possibly the ability to detect when you change page? Though would have to tell the programmer that they must insert a book for this functionality to work)
• Loom (Editing the displayed banner pattern, but only available if player or programmer puts a banner and some dye in first, also returning the selected banner pattern)
• Merchant (I'd like to be able to edit trades, but it's probably not possible)
• Shulker box (Nothing that I know of)
• Smithing (Maybe something with the new smithing templates? Nothing, it seems)
• Smoker (I'll redirect this to whatever the furnace does)
• Stonecutter (Selected recipe in the stonecutter, but only available if player or programmer puts some stone in first)
• Documentation (Dis gonna be chonky too)
• Add idiotproof checks to make sure programmer defines all the functions they need to etc.
• Add on_init function or smth like that for pages, so more complex initial setup can be done (useful for enchanting tables and beacons etc.)

[opsaaaaa]

I just read the code. This looks pretty cool.
I've been wanting to rework the waystone app, when I do, Ill try this out.
Unfortunately I may not have time to try it out for awhile.
But I think it should tick the boxes.
Thanks.

[Ghoulboy78]

55810ce
Recording this commit as last commit before removal of placeholder names, JIC something goes wrong with that and I need to revert.

[Ghoulboy78]

@altrisi I think this is ready

[rv3r]

Seems handy and the test script demonstrates some cool functions. Thanks for adding the example else reviewing this would take an eternity. I'll being going through this for a while but I don't expect any major errors.

[Ghoulboy78]

Thanks for adding the example else reviewing this would take an eternity.

I actually created it so I could test the script as I added more and more to it. Imaging what a nightmare it would have been otherwise...