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/material/list_test.go

@@ -5,7 +5,7 @@ import (
 	"testing"
 	"time"
 
-	"gioui.org/io/system"
+	"gioui.org/app"
 	"gioui.org/layout"
 	"gioui.org/op"
 	"gioui.org/unit"
@@ -15,7 +15,7 @@ import (
 
 func TestListAnchorStrategies(t *testing.T) {
 	var ops op.Ops
-	gtx := layout.NewContext(&ops, system.FrameEvent{
+	gtx := app.NewContext(&ops, app.FrameEvent{
 		Metric: unit.Metric{
 			PxPerDp: 1,
 			PxPerSp: 1,