Windows must be manually managed through the WaitFor function

Closes #5

manager.go

@@ -0,0 +1,33 @@
+package nasin
+
+import "git.tebibyte.media/tomo/tomo"
+import "git.tebibyte.media/tomo/tomo/event"
+
+var manager struct {
+	count int
+}
+
+type funcCookie func ()
+func (cookie funcCookie) Close () {
+	cookie()
+}
+
+// WaitFor ensures that the application will stay running while the given window
+// is open.
+func WaitFor (window tomo.Window) event.Cookie {
+	manager.count ++
+	
+	isManaged := true
+	handleClose := func () {
+		if !isManaged { return }
+		isManaged = false
+		manager.count --
+		if manager.count < 1 {
+			tomo.Stop()
+		}
+	}
+
+	return event.MultiCookie (
+		window.OnClose(handleClose),
+		funcCookie(handleClose))
+}