1. Fine tune the CSS styles, and add more examples
2. Add necessary "dimmer" animation for modal dialogs, otherwise the UI
seems flicking (follow #26469)
Replace #25446, fix#25438
All "cancel" buttons which do not have "type" should not submit the
form, should not be triggered by "Enter".
This is a complete fix for all modal dialogs.
The major change is "modules/aria/modal.js", "devtest" related code is
for demo/test purpose.