example/windows: add example that demonstrates multiple windows

Updates gio#19

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

example/windows/windows.go

@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package main
+
+// Multiple windows in Gio.
+
+import (
+	"log"
+
+	"gioui.org/app"
+	"gioui.org/io/event"
+	"gioui.org/io/system"
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/widget"
+	"gioui.org/widget/material"
+
+	"gioui.org/font/gofont"
+)
+
+type window struct {
+	btn widget.Clickable
+}
+
+func main() {
+	newWindow()
+	app.Main()
+}
+
+func newWindow() {
+	go func() {
+		w := new(window)
+		evts := app.NewWindow().Events()
+		if err := w.loop(evts); err != nil {
+			log.Fatal(err)
+		}
+	}()
+}
+
+func (w *window) loop(events <-chan event.Event) error {
+	th := material.NewTheme(gofont.Collection())
+	var ops op.Ops
+	for {
+		e := <-events
+		switch e := e.(type) {
+		case system.DestroyEvent:
+			return e.Err
+		case system.FrameEvent:
+			for w.btn.Clicked() {
+				newWindow()
+			}
+			gtx := layout.NewContext(&ops, e.Queue, e.Config, e.Size)
+			layout.Center.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
+				return material.Button(th, &w.btn, "More!").Layout(gtx)
+			})
+			e.Frame(gtx.Ops)
+		}
+	}
+}