NotificationServiceProvider accesses scene width and height only once

[AlmasB]

Describe the bug
If the game is then resized and auto scale is false, then notifications won't be centered.

Additional context / possible fixes
A fix is to slightly change internal API to bind to sceneService.prefWidthProperty()

Start from NotificationServiceProvider, L47.

FXGL/fxgl-ui/src/main/kotlin/com/almasb/fxgl/notification/impl/NotificationServiceProvider.kt

Lines 45 to 50 in 43a1a24

	private val notificationView by lazy {
	ReflectionUtils.newInstance(notificationViewClass).also {
	it.appWidth = sceneService.prefWidth.toInt()
	it.appHeight = sceneService.prefHeight.toInt()
	}
	}