Provide visible, keyboard-navigable button to show shortcuts help modal dialog

[cpcallen]

It should be possible for developers to add a visible button to their app that opens the shortcuts help modal, and which can be accessed via keyboard navigation. This plugin's test page should include such a button.

Possible approaches:

• Document API to call (from a normal form button outside the workspace)
• Provide an option that creates an on-workspace button in the style of the zoom controls.
  • (Can we even navigate to such controls at the moment?)

[cpcallen]

Per discussion with Micro:bit team: maybe this should be a general help button, not just a keyboard shortcuts help button.

[bmxedd]

The UI could be provided in a number ways:

• Always open the shortcut dialog, provide a link to external help within the dialog.
  • Shortcuts may not always be what the user wanted.
• Just open the help page in a new tab, shortcuts are a section within the help.
  • Multiple clicks to get shortcuts, if you just wanted a quick reference.
  • We also heard users wanted to have shortcuts as a reference side by side to their workspace. If the shortcuts were on an different page, this would enable that.
• Provide a menu which allows choice of shortcuts and direct help.
  • This can scale to having other items like sending feedback.
  • Example from gmail: