Support for backup UI backends

[Trigary]

Support for backup UI backends, eg. inventory, conversation API, Tom's ChatMenuAPI based UI "rendering" backends should be added. They should probably be treated the same way as Iodine GUIs. A function should exist that determines which backend to use for a given player. The backends must have a method that returns whether a specific player is supported.

The class linking the backends together should be able to open a new GUI instance for each player or reuse the existing instance. It should be kept in mind that while Iodine GUI instances can be shared and are synced between multiple players, other backends might not have that property.

[Trigary]

I now believe that this should be a separate project since it is just a wrapper around this one.