make it possible for non-entity-panels to call our echo method

This method is in charge to display a column after its argument in
Java, and nothing in scala or python.

Now, worldviews may call this method (eg when providing DnD
interactions)
cosmetics (properly use the echo method, and document what's going on)
recursion.hanoi: builtin getRadius(slot): radius of top-most disk
cosmetics in the changelog+we're gonna release v2.3
recursion.hanoi: allow to play with the mouse through drag and drop