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>

layout/list_test.go

@@ -70,7 +70,7 @@ func TestListPosition(t *testing.T) {
 		Constraints: Constraints{
 			Max: image.Pt(20, 10),
 		},
-		Queue: r,
+		Source: r,
 	}
 	el := func(gtx Context, idx int) Dimensions {
 		return Dimensions{Size: image.Pt(10, 10)}