context-sensitive F1 and change dark blue background color in help files

F1 opens a help page depending on the selection, or the word the cursor is
in/after.  Help page titles are used as the keywords, but
pre-titleization, so ":dull_bell" works to look up the "Dull Bell" help
page.  Rerun qt-doc.rb before building since struct help_page has changed
to add the keyword field.

Also, change dark blue background in docs so it doesn't match OSX selection color