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>

widget/button_test.go

@@ -6,9 +6,9 @@ import (
 	"image"
 	"testing"
 
+	"gioui.org/app"
 	"gioui.org/io/key"
 	"gioui.org/io/router"
-	"gioui.org/io/system"
 	"gioui.org/layout"
 	"gioui.org/op"
 	"gioui.org/widget"
@@ -21,7 +21,7 @@ func TestClickable(t *testing.T) {
 		b1  widget.Clickable
 		b2  widget.Clickable
 	)
-	gtx := layout.NewContext(&ops, system.FrameEvent{Queue: &r})
+	gtx := app.NewContext(&ops, app.FrameEvent{Queue: &r})
 	layout := func() {
 		b1.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 			return layout.Dimensions{Size: image.Pt(100, 100)}