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_android.go

@@ -650,6 +650,9 @@ func (w *window) ReadClipboard() {
 	})
 }
 
+// Close the window. Not implemented for Android.
+func (w *window) Close() {}
+
 // RunOnMain is the exported version of runOnMain without a JNI
 // environement.
 func RunOnMain(f func()) {