Improve keyboard handling.

* Prevents the space key from pushing buttons at the same time as advancing the game (caused the game to advance two phases at a time if the main button had focus).
* Sets focus in each modal.
* Disables game key handler whenever a modal is displayed instead of toggling a variable in only some cases.
* Blocks tabbing to control on the screen in the background whenever a modal is displayed.
* Adds a focus indicator so you can know where you're tabbing.