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_macos.go

@@ -198,7 +198,7 @@ func (w *window) draw(sync bool) {
 	cfg := configFor(w.ppdp, w.scale)
 	cfg.now = time.Now()
 	w.setStage(StageRunning)
-	w.w.event(UpdateEvent{
+	w.w.event(FrameEvent{
 		Size: image.Point{
 			X: width,
 			Y: height,