sweethome3d: Improvements

 -  Replace non freedesktop `CAD` desktop category
    by categories taken from debian's desktop item.

    This make the desktop items appear under the
    `Graphic` category in DE menus instead of `Others`.

 -  Moved to new package icons as specified by
    upstream release notes.

 -  Moved to icon name instead of full path to
    a single icon as specified by freedesktop
    specification.

 -  Human readable desktop item names.