Window currently in focus plus events

[BenjaminHarding]

Investigate having some events for when a window that a kiwi game resides on goes out of focus/comes into focus. Then can look at pausing the update loop/game timer/e.t.c.

[BenjaminHarding]

You can now add events for blur, focus, and visibility by accessing the appropriate Signals on the Stage. Also you get the visibility state of the game by accessing the visibility property on the Stage.

  //Signals
  this.game.stage.onFocus.add( this.inFocus, this );
  this.game.stage.onBlur.add( this.notInFocus, this );
  this.game.stage.onVisibilityChange.add( this.visiblityChanged, this );

  //....

  if( this.game.stage.visibility ) {
    //Resume game logic
  } else {
    //Pause the game
  }