WebPositive: fix cmd+esc handling

* Don't show the menu in MenusBeginning, this is invoked for all
shortcuts, so cmd+C and others would show the menu. Fixes #9186.
* Instead, show the interface specifically for the CMD+ESC case. Also,
send that key event to the default window handler so the menu opens.
Before, this was mistakenly sent to the "stop" button. Fixes #10523.