Keyboard buttons: Long press -> description

[chrisboyle]

Split out from #300: as well as the swap-L-R button, it would be useful for some of the others. Caution: long press on arrows, left-click and right-click buttons, and undo/redo, is already meaningful as repeat. So limit to the others? i.e. the game-specific actions that are letters on other platforms.

[tophatluke]

Something like the old help button from the Windows title bar could work, where after pressing a help menu item, you can press on an area of the screen for a description. This description could appear in the play field, so that users can quickly check multiple buttons' descriptions before returning to the game.