Re-focus a parent window when dialog closes

Fixes fyne-io#83

dialog/base.go

@@ -47,6 +47,8 @@ func (d *dialog) closed() {
 	if !d.responded && d.callback != nil {
 		d.callback(false)
 	}
+
+	d.win.RequestFocus()
 }
 
 func (d *dialog) setButtons(buttons fyne.CanvasObject) {

driver/gl/window.go

@@ -124,6 +124,12 @@ func (w *window) sizeOnScreen() (int, int) {
 	return viewWidth, viewHeight
 }
 
+func (w *window) RequestFocus() {
+	runOnMainAsync(func() {
+		w.viewport.Focus()
+	})
+}
+
 func (w *window) Resize(size fyne.Size) {
 	runOnMainAsync(func() {
 		scale := w.canvas.Scale()

test/testwindow.go

@@ -11,6 +11,7 @@ type testWindow struct {
 	fullScreen bool
 	fixedSize  bool
 	padded     bool
+	focused    bool
 	onClosed   func()
 
 	canvas    fyne.Canvas
@@ -44,6 +45,14 @@ func (w *testWindow) Resize(fyne.Size) {
 	// no-op
 }
 
+func (w *testWindow) RequestFocus() {
+	for _, win := range windows {
+		win.(*testWindow).focused = false
+	}
+
+	w.focused = true
+}
+
 func (w *testWindow) FixedSize() bool {
 	return w.fixedSize
 }
@@ -72,18 +81,23 @@ func (w *testWindow) SetOnClosed(closed func()) {
 	w.onClosed = closed
 }
 
-func (w *testWindow) Show() {}
+func (w *testWindow) Show() {
+	w.RequestFocus()
+}
 
 func (w *testWindow) Clipboard() fyne.Clipboard {
 	return w.clipboard
 }
 
-func (w *testWindow) Hide() {}
+func (w *testWindow) Hide() {
+	w.focused = false
+}
 
 func (w *testWindow) Close() {
 	if w.onClosed != nil {
 		w.onClosed()
 	}
+	w.focused = false
 
 	windowsMutex.Lock()
 	i := 0

window.go

@@ -20,6 +20,11 @@ type Window interface {
 	// platform constraints.
 	Resize(Size)
 
+	// RequestFocus attempts to raise and focus this window.
+	// This should only be called when you are sure the user would want this window
+	// to steal focus from any current focused window.
+	RequestFocus()
+
 	// FixedSize returns whether or not this window should disable resizing.
 	FixedSize() bool
 	// SetFixedSize sets a hint that states whether the window should be a fixed