Make holdings scrollable

Make holdings scrollable

When the hand size exceeds the number of board ranks R, the holdings will
either display either the first R-1 or last R-1 piece types. The remaining
square will then act as a toggle button between the two states. If there
is indeed material that is not displayed, the toggle square will display
a Prince of the opposite color. (The idea is that themes for games needing
this feature would define this piece as a vertical arrow glyph.)