[FEATURE] GUI Trading

[ax-ie]

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FEATURE:
https://cdn.discordapp.com/attachments/884336757418651688/884686144883814430/2021-09-07_07-25-58.mp4
in the gui on a sign it shows how many of what item your trading like in my case it would say 1 diamond on the sign then when you clicked what you wanted it would take the 1 diamond and replace it with the cobblestone and give you the cobblestone

[KillerOfPie]

@SparklingComet I asked this to be put here and will look into it when appropriate.

[KillerOfPie]

I'm planning on adding this(with some changes) to 2.7.0. Below are the changes and some addition things that may also be added after RocketManKian#8769 and I discussed things on Discord.

1. There will be (at least) 4 new GUI style shops; GUI-Trade, GUI-BiTrade, GUI-ITrade, GUI-OrTrade(new type)
2. When clicked these shops will open a GUI similar to the what menu.
   For Trade, BiTrade, and ITrade: Clicking on the new Trade Button in this menu will perform the trade and there will be a multiplier icon that can be increased or decreased to change your multiplier. To trade using the multiplier you will need to click on the multiply icon.
   For OrTrade: You would first see a what screen with available cost groups, upon selecting a cost group the above gui would open with that group as the cost. (Not sure yet how storing this would be best(Set of Sets, Map<groupID, Item set>, Adding and Addition Set on top of the existing Set so that changing the existing sets and handling the change in load is unnecessary ... 1 may be the best option if we replace the existing objects, otherwise 3.
3. This may be made as an Add-On
   ^ If Add-On I may make some internal changes to allow new signs and settings to be registered with the plugin instead of using Enums for all of them(which limit them being Extended, Extending, and being Expanded upon...

@SparklingComet Input?
The problem I see with it being a Add-on is the changes that would need to be made to the shop Object or to saving the Objects so that they can be re-loaded as a child Object(which may cause other problems if the Add-on is removed)