// add a button to a scene
// similar to buttons in Phaser v2
Phaser.Scene.prototype.addButton = function(x, y, key, callback, callbackContext, overFrame, outFrame, downFrame, upFrame)
{
		// add a button
		var btn = this.add.sprite(x, y, key, outFrame).setInteractive();
		btn.on('pointerover', function (ptr, x, y) { this.setFrame(overFrame) } );
		btn.on('pointerout',  function (ptr)       { this.setFrame(outFrame) } );
		btn.on('pointerdown', function (ptr)       { this.setScale(0.9, 0.9) } );
		btn.on('pointerup', callback.bind(callbackContext));
		
		return btn;
};