io/system,app: [API] move DestroyEvent, StageEvent, Stage to package app

They're only useful at the top-level event loop in combination with an
app.Window.

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

io/system/system.go

@@ -1,51 +0,0 @@
-// SPDX-License-Identifier: Unlicense OR MIT
-
-// Package system contains events usually handled at the top-level
-// program level.
-package system
-
-// DestroyEvent is the last event sent through
-// a window event channel.
-type DestroyEvent struct {
-	// Err is nil for normal window closures. If a
-	// window is prematurely closed, Err is the cause.
-	Err error
-}
-
-// A StageEvent is generated whenever the stage of a
-// Window changes.
-type StageEvent struct {
-	Stage Stage
-}
-
-// Stage of a Window.
-type Stage uint8
-
-const (
-	// StagePaused is the stage for windows that have no on-screen representation.
-	// Paused windows don't receive frames.
-	StagePaused Stage = iota
-	// StageInactive is the stage for windows that are visible, but not active.
-	// Inactive windows receive frames.
-	StageInactive
-	// StageRunning is for active and visible Windows.
-	// Running windows receive frames.
-	StageRunning
-)
-
-// String implements fmt.Stringer.
-func (l Stage) String() string {
-	switch l {
-	case StagePaused:
-		return "StagePaused"
-	case StageInactive:
-		return "StageInactive"
-	case StageRunning:
-		return "StageRunning"
-	default:
-		panic("unexpected Stage value")
-	}
-}
-
-func (StageEvent) ImplementsEvent()   {}
-func (DestroyEvent) ImplementsEvent() {}