app{,/internal/window}: make app.Main blocking on desktop platforms

This CL implements the app.Main function as a blocking-forever function
for JS, Wayland, Windows and X11.
This works better for applications that can now programmatically close
windows.

app/internal/window/os_wayland.go

@@ -232,14 +232,8 @@ func newWLWindow(window Callbacks, opts *Options) error {
 		d.destroy()
 		return err
 	}
-	// Increment window counter.
-	windowCounter <- +1
 	w.w = window
 	go func() {
-		defer func() {
-			// Decrement window counter.
-			windowCounter <- -1
-		}()
 		defer d.destroy()
 		defer w.destroy()
 		w.w.SetDriver(w)