Refactoring counted Map<T, Integer> as Multiset<T>

Guava Multiset already handles adding/removing Amount for us, so we should not need to do that ourselves.

Big uses of `, Integer>` i have seen and could be changed:
* `ItemPool implements Iterable>` (and it uses `Map.Entry`)
* `Map` in GameEntity (and Subclasses) #11121
* `ActivationTable` is a `Table, List>`, but the Value part should be a `Multiset` instead #10998

What probably shouldn't be changed:
* Damage noticed on a Card using `Map`

undecided:
* `TokenCreateTable extends ForwardingTable`
in the end, the prototype is copied and giving a new ID each