skipping missions in campain possible

[reported by Marcel Sebek <sebek64@post.cz> to the
Debian gl-117 package ]

Package: gl-117
Version: 1.3.0-2
Severity: normal

I've found way how to play particular mission in
campain without completing previous missions. Buttons
with mission names is hidden after processing clicks on
them. So when you finish a mission, double click on
right side of "Return to main menu" allows to play one
of the "moon" missions.