Set default watermark value for current theme
Examples
# Set default watermark using a character
setDefaultWatermark("Confidential")
addScatter(x = c(1, 2, 1, 2, 3), y = c(5, 0, 2, 3, 4))
# Set default watermark using a `Label` object
setDefaultWatermark(Label$new(text = "Confidential", color = "red", angle = 30))
addScatter(x = c(1, 2, 1, 2, 3), y = c(5, 0, 2, 3, 4))