[Controls, Spinner] Add key binding to increment and decrement the Spinner

Right now the only way I see to increment and decrement is using the mouse and clicking on the arrow buttons. Some keybinding would make this much more friendly.
I would think that up arrow/down arrow is best.
That said, the buttons can point left and arrow (depending on the css style), so maybe follow that. This way it would be consistent with SplitButton/MenuButton, (but bad for the a11y stand point).