Send HELP_KEYBINDINGS event instead of ANY_INPUT event after displaying help screen

If input_context consumers were registered to the special ANY_INPUT event and
the HELP_KEYBINDINGS event, they would receive an ANY_INPUT event when the
user fired the HELP_KEYBINDINGS event.

Instead, send a HELP_KEYBINDINGS event back to the caller. (It is useful to send the HELP_KEYBINDINGS event to callers so they can redraw the screen after the help window disappears, which would otherwise leave an unpainted region)