app,app/internal/window: [macOS] add app.Window.Close for closing a window

Recently support was added for multiple top-level windows. Add support
for closing those windows.

macOS only; all others stubbed out.

Signed-off-by: Larry Clapp <larry@theclapp.org>

app/internal/window/os_x11.go

@@ -110,6 +110,9 @@ func (w *x11Window) WriteClipboard(s string) {
 
 func (w *x11Window) ShowTextInput(show bool) {}
 
+// Close the window. Not implemented for X11.
+func (w *x11Window) Close() {}
+
 var x11OneByte = make([]byte, 1)
 
 func (w *x11Window) wakeup() {