Allow keyboard control

- [ ] Ready to merge

The start of making JS Bin fully keyboard accessible.

- Restore focus style
- Dynamically change tabindex depending on whether the menu (or welcome panel) is open or not
- Allow F9 to focus *out* of editor to the panel selection tool (by default it focuses to the first panel button: html) - fixes #1936 
- WIP

- [ ] Aria roles and states
- [ ] Solution for menu items with input elements