Fix dropdown menu accessibility issues

[microbit-robert]

This PR addresses a few accessibility issues with the react-common MenuDropdown component.

• Fixes the accessibility tree by removing wrapping li and ul with roles of "none" and "group". This was causing keyboard navigation with NVDA to get stuck in the menu and VoiceOver to read every menuitem as 1 of 1. Accompanied by minor CSS change to avoid the appearance of a horizontal scroll bar
• tags are now wrapped in an
• and both are given appropriate roles
• The first menuitem is focused when opened via keyboard (mouse behaviour is not affected)
• Adds new onClickEvent prop to Button which takes the MouseEvent - required to determine whether the menu was opened by mouse of keyboard. I have chosen not to change the existing onClick event to avoid a breaking change across the targets.