ui/app: rename DrawEvent to UpdateEvent and Window.Draw to Window.Update

Window.Draw is not the right name for a method that does more than
just drawing. Rename to Update instead, and rename the DrawEvent
accordingly for symmetry.

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

ui/app/os_android.go

@@ -300,7 +300,7 @@ func (w *window) draw(sync bool) {
 		return
 	}
 	ppdp := float32(w.dpi) * inchPrDp
-	w.event(DrawEvent{
+	w.event(UpdateEvent{
 		Size: image.Point{
 			X: int(width),
 			Y: int(height),