Fixes #35338

Changes proposed in this PR:

• Fixes keyboard accessibility of Dropdown component, adding missing aria attributes and simplifying focus handling:
  • When closing the menu, focus now always returns to the button that opened the menu as per this WCAG Example implementation
• Makes the "More" menu item take up the whole width of the menu, increasing its click target
• Adds a placement prop to customise the dropdown menu's position per component
• Removes deprecated onKeyPress prop from IconButton which was only used by the Dropdown component and is not needed anymore now since onClick handles keyboard interactions fine

Screencap

This video shows the menu being opened by Enter key first, then Space, then by clicking on the empty space to the right of the "More" button. Each time the menu is navigated briefly and closed via Esc