Custom model data

[minehostil]

Add support for custom model data to menu items and page arrows; here is an example
gui:
reward-locked:
free: GRAY_STAINED_GLASS
custom-model-data: 101
premium: GRAY_STAINED_GLASS
custom-model-data: 102
next-page:
material: arrow
custom-model-data: 110
prev-page: arrow
custom-model-data: 110