fix Linux desktop integration

[LuckyTurtleDev]

• change the Gcode viewer .desktop file to SuperSlicer (original file did open Prusa Slicer)
• rename prusa-gcodeviewer to supergcodeviewer to avoid conflicts with Prusa Slicer
• move install directory of .desktop files from ${SLIC3R_FHS_RESOURCES}/applications) to ${CMAKE_INSTALL_FULL_DATAROOTDIR}/applications
  Linux desktops search in /usr/share/applications and not in /usr/share/SuperSlicer/applications for .desktop files
• install .desktop icons to /usr/share/pixmaps
  otherwise the .desktop files shows no icons, because the icons can not be found
• add description to the .desktop files

Furthermore I think this symlink is not a good idea. If you execute prusa-gcodeviewer SuperSlicer will starts normal and not in the gcode viewer mode. May using a script is a better Solution.
As example for Linux can this little script been used:

#!/bin/bash
superslicer --gcodeviewer "$@"