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_wayland.go

@@ -1426,6 +1426,9 @@ func (w *window) surface() (*C.struct_wl_surface, int, int) {
 
 func (w *window) ShowTextInput(show bool) {}
 
+// Close the window. Not implemented for Wayland.
+func (w *window) Close() {}
+
 // detectUIScale reports the system UI scale, or 1.0 if it fails.
 func detectUIScale() float32 {
 	// TODO: What about other window environments?