Don't use Null Game's extra source files for all GUI programs

All the not-quite-puzzle GUI programs (only galaxieseditor at this
point) were using the extra source files from Null Game, presumably as a
convenient way of asking not to have an icon on platforms where icons
are provided as extra source files.  Unfortunately, this all went wrong
if Null Game did have a save file for an icon, causing CMake to complain
about multiple definitions of a target.

Now these programs look for extra files under their own names, which
will work just as well since those have no save files to generate icons
from either.

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