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_x11.go

@@ -458,7 +458,7 @@ loop:
 
 		if (anim || syn) && w.config.Size.X != 0 && w.config.Size.Y != 0 {
 			w.w.Event(frameEvent{
-				FrameEvent: system.FrameEvent{
+				FrameEvent: FrameEvent{
 					Now:    time.Now(),
 					Size:   w.config.Size,
 					Metric: w.metric,