layout,app: [API] rename FrameEvent.Queue and Context.Queue to Source

We're about to replace the interface Queue with a concrete input.Source.
This change renames the field accordingly.

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

widget/selectable_test.go

@@ -38,7 +38,7 @@ func TestSelectableMove(t *testing.T) {
 	gtx := layout.Context{
 		Ops:    new(op.Ops),
 		Locale: english,
-		Queue:  r,
+		Source: r,
 	}
 	cache := text.NewShaper(text.NoSystemFonts(), text.WithCollection(gofont.Collection()))
 	fnt := font.Font{}