Allow plugins to register help links

[ray-millward-tessella]

Description:
The help icon in the main app bar should become a menu with links to the help locations (probably a different documentation site) - as such, plugins should also be able to fire a message to the parent app in order to register a link in this menu to their own documentation.

Acceptance criteria:

• Clicking the help icon in the main app bar opens a menu with a list of links to the help pages
• Plugins can fire a message back to the parent that adds a new item to the help menu.