Change herbalist tables' display of fullnes

[Potjeh]

Right now, herbalist tables show herb graphics based on the number of items that are sprouting/hatching/whatever on it. Change this so the base is the number of slots taken by those items. The advantages of that are the following:
- you can tell at a glance which tables have free room
- it's easier to notice when your trees sprout
- it won't drive us OCD types mad