modal property Null safety
True if the ShortcutManager should not pass on keys that it doesn't handle to any key-handling widgets that are ancestors to this one.
Setting modal to true will prevent any key event given to this manager from being given to any ancestor managers, even if that key doesn't appear in the shortcuts map.
The net effect of setting modal to true is to return KeyEventResult.skipRemainingHandlers from handleKeypress if it does not exist in the shortcut map, instead of returning KeyEventResult.ignored.
final bool modal;