app,io/system: extract system events to separate package

Package app is the only package that depends on native libraries and
Cgo. Minimize its API, thereby minimizing Gio clients' dependency on
it. In the future, a headless, testing or remote "Window" should be
very easy to replace app.Window.

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

layout/layout.go

@@ -4,9 +4,9 @@ package layout
 
 import (
 	"image"
-	"time"
 
 	"gioui.org/io/event"
+	"gioui.org/io/system"
 	"gioui.org/op"
 	"gioui.org/unit"
 )
@@ -53,20 +53,11 @@ type Context struct {
 	// operation.
 	Dimensions Dimensions
 
-	Config
+	system.Config
 	event.Queue
 	*op.Ops
 }
 
-// Config define the essential properties of
-// the environment.
-type Config interface {
-	// Now returns the current animation time.
-	Now() time.Time
-
-	unit.Converter
-}
-
 const (
 	Start Alignment = iota
 	End
@@ -104,7 +95,7 @@ func (s *Context) Layout(cs Constraints, w Widget) Dimensions {
 
 // Reset the context. The constraints' minimum and maximum values are
 // set to the size.
-func (c *Context) Reset(cfg Config, size image.Point) {
+func (c *Context) Reset(cfg system.Config, size image.Point) {
 	c.Constraints = RigidConstraints(size)
 	c.Dimensions = Dimensions{}
 	c.Config = cfg