Fix focus issues in Lumino menu bar

[gabalafou]

Summary

A cursor is to a mouse user what focus is to a keyboard user.

The Lumino menu bar has a number of focus anti-patterns, which makes it very hard to use if you are a blind or low-vision keyboard user, such as: moving focus on mouse over (you generally should not shift browser focus unless a key has been pressed or the mouse has been clicked), not moving browser focus as the user navigates the menu with a keyboard, and others.

Acceptance Criteria

• The JupyterLab focus visible test passes
• Focus does not move when user moves mouse over menu bar item
• Focus does move when user uses keyboard to select item in menu bar
• Pressing escape does not lose focus to the document body

Tasks to complete

• Watch PhosphorJS mentor sessions to better understand the code architecture and philosophy of Lumino
• Submit PR that resolves active state versus focus state
• Maybe: try to get Phosphor author to look at PR
• Merge PR

[gabalafou]

Note, the one part of the acceptance criteria that jupyterlab/lumino#607 does not meet is:

Pressing escape does not lose focus to the document body