Button: splitbutton demo accessibility issues

1. The button that activates the menu should have aria-haspopup="true" so that the screen reader user is aware of the behavior

2. When activating the button, the focus should be placed into the popup menu

3. The anchors inside the menu should not be naturally tab focusable (set tabindex=-1)

4. Tabbing out of the menu should close the menu

5. ESC should close the menu and set focus back onto the button

6. When the menu is up, button should have aria-pressed="true"