Add context sensitive help

[herzbube]

With the new user manual there exists now the possibility to define anchors in the user manual HTML code which can be used to navigate directly to specific user manual content.

This opens up the possibility for providing context sensitive help to the UI. Idea how it could work:

• The UI contains a new "Context help" button.
• The user taps the button.
• The next thing that the user touches is what the user wants help for.
• The app determines some sort of context or id of the thing that was touched.
• The app looks up the user manual URL that corresponds to the context or id in some internal dictionary.
• The app opens the user manual and instructs UserManualViewController to navigate to the URL.

Not yet known: How to attach and retrieve a context or id to/from user interface elements.