F10 set focus to main menu.

F10 set focus to main menu,
After main menu got focus,  Down or Up arrows make main menu open.

Next TODO is to move focus to next menu item when arrow key is pressed.

fixes https://github.com/dart-lang/spark/issues/1042, https://github.com/dart-lang/spark/issues/924, https://github.com/dart-lang/spark/issues/744

review
 @devoncarew for `actions.dart`, `spark.dart`.
 @ussuri for `widgets`.

TEST=Press F10 -> Press DOWN arrow key -> Check main menu is open.