Add convenience factory methods for Border and Background

It's lengthy to create simple borders and backgrounds:

new Border(new BorderStroke(Color. BLACK, BorderStrokeStyle.SOLID, null, null));
new Background(new BackgroundFill(Color.BLACK, null, null));

Adding convenience factory methods can make this easier:

Border.of(Paint stroke), or Border.stroke(Paint stroke)
Background.of((Paint fill), or Background.fill(Paint fill)