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_ios.go

@@ -299,6 +299,9 @@ func (w *window) ShowTextInput(show bool) {
 	})
 }
 
+// Close the window. Not implemented for iOS.
+func (w *window) Close() {}
+
 func NewWindow(win Callbacks, opts *Options) error {
 	mainWindow.in <- windowAndOptions{win, opts}
 	return <-mainWindow.errs