improve GUI feel a bit

- Now can only click to intend to move player-on-turn's pieces.
- If you click several of them in turn, it's not taken to be an
  (illegal) move, but a re-decision about which piece to move.
refactor: name all function expressions

This is useful in stacktraces, because the names show up as actual names and
not as <anon> or something. Easier to see exactly what the stack is made of.