Solitaire and Spider have almost identical code for dragging cards from
one stack to another, so it makes sense to move that here. But using
the term "moving" for them, instead of "focused" which (to me at least)
was not clear what it meant.
As part of this, made a const overload for `Card::rect()`. We need the
non-const one too as it's used for modifying the position of a card
that's being dragged. I plan on changing that soon but we'll see.
For now, the only feature of this is that it sets the background colour
from the `Games::Cards::BackgroundColor` config value. Later, other
card game configuration and shared behaviour can go here, to save
duplicating it in each game.
