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/doc.go

@@ -13,7 +13,7 @@ is embedded in another project, NewWindow merely connects with a previously
 created window.
 
 A Window is run by receiving events from its Events channel. The most
-important event is UpdateEvent that prompts an update of the window
+important event is FrameEvent that prompts an update of the window
 contents and state.
 
 For example:
@@ -22,7 +22,7 @@ For example:
 
 	w := app.NewWindow()
 	for e := range w.Events() {
-		if e, ok := e.(app.UpdateEvent); ok {
+		if e, ok := e.(app.FrameEvent); ok {
 			ops.Reset()
 			// Add operations to ops.
 			...
@@ -60,7 +60,7 @@ For example, to display a blank but otherwise functional window:
 Event queue
 
 A Window's Queue method returns an event.Queue implementation that distributes
-incoming events to the event handlers declared in the latest call to Update.
+incoming events to the event handlers declared in the latest frame.
 See the gioui.org/ui package for more information about event handlers.
 
 */