Fix Sys_Dialog for *nix systems not using gnome or KDE
As the title says ... there is no implementation for others.
On systems not using KDE or Gnome, xmessage is used. This includes *BSDs as they use X11 like linux distributions.
GNOME_DESKTOP_SESSION_ID is obsolete.
- Status changed from New to Fixed
- % Done changed from 0 to 100
- Target version changed from 2.75 to 2.71rc2
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