app: rename UpdateEvent to FrameEvent and add Frame field

While "DrawEvent" was too specific (op.Ops contains non-draw events),
"Update" is too vague: it's a common word, and could be misunderstood
to mean update parts of a window, not replace it.

"FrameEvent" is more specific, and is the usual way to refer to immediate
mode drawing.

While we're here, unexport Window.Update and add a Frame function to
FrameEvent, to emphasize that updating the window frame is only
appropriate during the handling of a FrameEvent.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/os_ios.go

@@ -80,7 +80,7 @@ func onDraw(view C.CFTypeRef, dpi, sdpi, width, height C.CGFloat, sync C.int, to
 	if sync != 0 {
 		isSync = true
 	}
-	w.w.event(UpdateEvent{
+	w.w.event(FrameEvent{
 		Size: image.Point{
 			X: int(width + .5),
 			Y: int(height + .5),