Piece selector customisation

Currently the piece selector is kind of the weakest point in the package in terms of customisability.

We probably want to support a few things:
* Customising the appearance of the built in piece selector (basic stuff like square colours, borders, etc)
* Horizontal orientation
* Anchoring the selector in the centre of the board instead of on the square (for promo only I guess?)
* Padding, different spacing or arrangement of pieces
* Maybe the ability to totally override the show piece selector functionality in `BoardController` with custom logic (for cases where you might want to show a modal, for example, or for custom game logic)

It could just be that we allow overriding the piece selector building functionality entirely, but there's a lot of logic there and that it's probably worth coming up with a better solution. Maybe `PieceSelector` should be a more modular design that accepts a builder function with a list of piece widgets (how to allow overriding their size without making it complex?) and the theme etc