Unify stacked bookshelf inventories when aligned

[TheAtomicOption]

The bookcases in Macaw's Furniture combine visually when stacked with the same facing, but the inventories remain separate. This both makes it a bit awkward to target a specific bookcase, and doesn't match the expectation from vanilla chests that the inventories combine when the models combine.

I think the ideal unified inventory would:

• show all the inventories for the column of aligned bookshelves stacked in the same vertical order as the bookshelves,
• have a small line between each shelf's inventory (maybe the line is the same color as the bookshelf's wood color, but leave the background grey for visibility of the contents)
• have a scrollbar if there are too many combined inventories for the screen height.
• when opening a combined bookshelf inventory of a stack large enough to have a scrollbar, the clicked shelf's inventory is scrolled into view on open.