Async ItemStack update

[weihao]

Is your feature request related to a problem? Please describe.

Created a page full of player skulls, whenever the player opens the GUI for the first time, the server freezes.

Describe the solution you'd like

Asynchronous action on GUI icon itemstack after GUI is opened.

Describe alternatives you've considered

Additional context

[Phoenix616]

Accessing the Bukkit API async is in general not a good idea so I'm not too sure if this can be supported directly in the library, I suggest completing the player profiles of the skull async before adding them to the GUI.