Added missing MouseEvent coordinates

[toddharrison]

Specifically pageX, pageY, offsetX, and offsetY.

[toddharrison]

This is needed for identifying the point clicked on a canvas, for example.

val map = canvas(width = 200, height = 200) { it["class"] = "map" }
val label = span().text("No selection")
map.on.click { event ->
    label.text("Clicked (${event.offsetX}, ${event.offsetY}) on the Canvas")
}