Items should be stackable

[jrmi]

Sometimes you want some items to "hold" items stacked on it and when you move these items, all items on top of it should move at the same time.

It should also be possible to configure items to be stackable on items of the same family and if so, it should be possible to configure the position delta between each element of the stack. For instance you could stack tokens to make small towers.

The number of stacked item could be display in some way if necessary?

Could we use the stack action each time we add a new stacked item on top of the pile?

It's related to #345