Display captured pieces

[lightvector]

Add a simple display for captured pieces to the game client. Currently there is none, so it's sometimes tricky to tell the material balance at a glance.

[clyring]

Before the suggestion from #117 of using the captured piece area to allow for a more natural setup interface, I would have considered this a very low priority, if probably also low difficulty. I don't think a visual material-o-meter is very useful, generally speaking, and it is often outright counterproductive in my experience to look at one in positions that have had lots of exchanges. What's on the board is much more important than what's off the board. But I'm okay with notching this up a bit in priority as a stepping stone toward other improvements, and would be philosophically pleased by progress toward a setup interface that does not suggest any particular setups.

Generically I might have slightly preferred handicap games to also allow placing more than a full army as well, but on thinking of use cases I'm more than fine with delegating such situations to directSetup games.