Phantom slot mechanic

This mechanic is used to specify items in some of Fine Stuff GUI slots.
 * Setting an item: take a stack and click it on the slot. The slot will get a copy of the stack.
 * Increasing/decreasing stack size by 1: click/right click on the slot with empty cursor.
 * Increasing/decreasing stack size by 10: Shift + click/right click on the slot with empty cursor.
 * Increasing/decreasing stack size by arbitrary number: take a stack of same item type with desired size and click/right click on the slot. The phantom stack size will be increased/decreased by the held stack size.
 * Removing the phantom stack: middle click on the slot with empty cursor.