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.go

@@ -132,7 +132,7 @@ func (o Orientation) String() string {
 }
 
 type frameEvent struct {
-	system.FrameEvent
+	FrameEvent
 
 	Sync bool
 }