Change XDG desktop entry to only run single instance of FreeCAD.

Change XDG desktop entry to only run single instance of FreeCAD.

This ensure invoking it to open different files only start the
program once, and load new files into the already running instance.
 Related to https://bugs.debian.org/1029846 and
 https://github.com/FreeCAD/FreeCAD/commit/c7a21ecbeecefe7c2dfc9e950b3d6bb42351d476 .

 Partly applied upstream.  Arguments applied, program name change is not.
Author: Håvard Flaget Aasen <haavard_aasen@yahoo.no>
Forwarded: https://github.com/FreeCAD/FreeCAD/pull/9855
Last-Update: 2021-12-26

Gbp-Pq: Name 1100-run_single-instance.patch