widget: [API] convert Editor to return one event at a time

This commit eliminates (*widget.Editor).Events() in favor of making
(*widget.Editor).Update() return events as they are generated in response to
input. This makes the behavior of the editor match the rest of the core widgets.
Callers who previously invoked Events() can now achieve the same thing by using
a loop like this:

for {
	ev, ok := editor.Update(gtx)
	if !ok {
		break
    }
	// Handle ev
}

This is undeniably more verbose, but it enables more sophisticated event processing.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/editor_test.go

@@ -905,7 +905,6 @@ g 2 4 6 8 g
 		gtx.Execute(key.FocusCmd{Tag: e})
 		// Layout once with no events; populate e.lines.
 		e.Layout(gtx, cache, font, fontSize, op.CallOp{}, op.CallOp{})
-		e.Events() // throw away any events from this layout
 
 		r.Frame(gtx.Ops)
 		gtx.Source = r.Source()
@@ -929,14 +928,13 @@ g 2 4 6 8 g
 		)
 		tim += time.Second // Avoid multi-clicks.
 
-		e.Layout(gtx, cache, font, fontSize, op.CallOp{}, op.CallOp{})
-		for _, evt := range e.Events() {
-			switch evt.(type) {
-			case SelectEvent:
-				return e.SelectedText()
+		for {
+			_, ok := e.Update(gtx) // throw away any events from this layout
+			if !ok {
+				break
 			}
 		}
-		return ""
+		return e.SelectedText()
 	}
 	type screenPos image.Point
 	logicalPosMatch := func(t *testing.T, n int, label string, expected screenPos, actual combinedPos) {
@@ -1162,12 +1160,18 @@ func TestEditor_Submit(t *testing.T) {
 	r.Queue(
 		key.EditEvent{Range: key.Range{Start: 0, End: 0}, Text: "ab1\n"},
 	)
-	e.Layout(gtx, cache, font, fontSize, op.CallOp{}, op.CallOp{})
 
+	got := []EditorEvent{}
+	for {
+		ev, ok := e.Update(gtx)
+		if !ok {
+			break
+		}
+		got = append(got, ev)
+	}
 	if got, want := e.Text(), "ab1"; got != want {
 		t.Errorf("editor failed to filter newline")
 	}
-	got := e.Events()
 	want := []EditorEvent{
 		ChangeEvent{},
 		SubmitEvent{Text: e.Text()},