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>

layout/context.go

@@ -3,7 +3,6 @@
 package layout
 
 import (
-	"image"
 	"time"
 
 	"gioui.org/io/event"
@@ -35,43 +34,6 @@ type Context struct {
 	*op.Ops
 }
 
-// NewContext is a shorthand for
-//
-//	Context{
-//	  Ops: ops,
-//	  Now: e.Now,
-//	  Queue: e.Queue,
-//	  Config: e.Config,
-//	  Constraints: Exact(e.Size),
-//	}
-//
-// NewContext calls ops.Reset and adjusts ops for e.Insets.
-func NewContext(ops *op.Ops, e system.FrameEvent) Context {
-	ops.Reset()
-
-	size := e.Size
-
-	if e.Insets != (system.Insets{}) {
-		left := e.Metric.Dp(e.Insets.Left)
-		top := e.Metric.Dp(e.Insets.Top)
-		op.Offset(image.Point{
-			X: left,
-			Y: top,
-		}).Add(ops)
-
-		size.X -= left + e.Metric.Dp(e.Insets.Right)
-		size.Y -= top + e.Metric.Dp(e.Insets.Bottom)
-	}
-
-	return Context{
-		Ops:         ops,
-		Now:         e.Now,
-		Queue:       e.Queue,
-		Metric:      e.Metric,
-		Constraints: Exact(size),
-	}
-}
-
 // Dp converts v to pixels.
 func (c Context) Dp(v unit.Dp) int {
 	return c.Metric.Dp(v)