Add DesktopNames to xdg desktop file.

  This is used with xdg-desktop-portal, and will allow downstream
  distros/users to create fvwm3-portals.conf or fvwm-portals.conf
  to configure xdg-desktop-portal for fvwm3.

contrib/xsession/fvwm3.desktop.in

@@ -4,4 +4,5 @@ Keywords=Fvwm3 Window Manager
 Comment=F? Virtual Window Manager
 Icon=@datadir@/fvwm3/default-config/images/fvwm-logo-small.png
 Type=Application
+DesktopNames=FVWM3;FVWM;
 Exec=@bindir@/fvwm3