Add a help page to many screens

This adds a help page for some screens that explains what the screen does. There are details of what key presses are supported by the screen and how to use it. There may be details of any known problems, a Frequently Asked Questions section and what's on the todo list for future improvements.

You can see the help by pressing HELP (H) or choose the Help... option from the popup menu available on some screens.

A screenshot of the help screen for the WebCamViewer.