Basically a double chest-like UI that would list all the maps the user has.
Taking a map from it would actually give the user a copy of this map, as if requested using /maptool get.
Right-clicking a poster map would open another GUI that lists all the map parts of the poster, arranged like they would be in the world (i.e. using their rows and columns, if this data is available).
Putting a map in the inventory would remove (or refund) it, but putting any other item would simply cancel the event.

If the amount of maps is too large for the GUI to show, scrollbar-like arrows will appear on the sides of the GUI (vertical only for the global map GUI, both if necessary for the poster map GUI) :

OOOOOOOO^
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOOv
<       >

(O = inventory slot)

Arrow hover-text would show the total amount of pages as well (example : "Go to page 2 of 9").

Closed by #19.