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/doc.go

@@ -17,12 +17,10 @@ FrameEvent that prompts an update of the window contents.
 
 For example:
 
-	import "gioui.org/unit"
-
 	w := app.NewWindow()
 	for {
 		e := w.NextEvent()
-		if e, ok := e.(system.FrameEvent); ok {
+		if e, ok := e.(app.FrameEvent); ok {
 			ops.Reset()
 			// Add operations to ops.
 			...