Make tutorial dialog a proper window, clean-up menu.
Show welcome message to new user when tutorial available.
Repeat current tutorial message, when no user progress.
$FG_ROOT/gui/dialogs/tutorial.xml. It combines the former selection
and description dialogs, which makes browsing much more pleasant.
No more switching back and forth between the two old dialogs. Also,
the <list> box can handle a bigger number of tutorials than the
<combo> box could.
- remove comments from the top of tutorial.nas. The documentation is
now $FG_ROOT/Docs/README.tutorials
