app,io/system,layout: [API] move FrameEvent and Insets to package app

In the early days of Gio, FrameEvent was part of package app. It was
moved to package system to enable layout.NewContext be a convenient
short-hand for constructing a layout.

However, it seems the better design to leave FrameEvent (and Insets) in
package app, and move layout.NewContext there as well. More importantly,
the move allows us to replace the event.Queue interface with a concrete
type.

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

app/os_macos.go

@@ -777,7 +777,7 @@ func (w *window) draw() {
 	cfg := configFor(w.scale)
 	w.setStage(system.StageRunning)
 	w.w.Event(frameEvent{
-		FrameEvent: system.FrameEvent{
+		FrameEvent: FrameEvent{
 			Now:    time.Now(),
 			Size:   w.config.Size,
 			Metric: cfg,