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_js.go

@@ -666,7 +666,7 @@ func (w *window) draw(sync bool) {
 	}
 
 	w.w.Event(frameEvent{
-		FrameEvent: system.FrameEvent{
+		FrameEvent: FrameEvent{
 			Now:    time.Now(),
 			Size:   size,
 			Insets: insets,
@@ -676,10 +676,10 @@ func (w *window) draw(sync bool) {
 	})
 }
 
-func (w *window) getConfig() (image.Point, system.Insets, unit.Metric) {
+func (w *window) getConfig() (image.Point, Insets, unit.Metric) {
 	invscale := unit.Dp(1. / w.scale)
 	return image.Pt(w.config.Size.X, w.config.Size.Y),
-		system.Insets{
+		Insets{
 			Bottom: unit.Dp(w.inset.Y) * invscale,
 			Right:  unit.Dp(w.inset.X) * invscale,
 		}, unit.Metric{