[Tools] extend ThumbNail Provider for .FCBak
[Tools] extend ThumbNail Provider for .FCBak

- as requested in the forum, the thumbnail provider should also work for *.FCBak file, see
https://forum.freecadweb.org/viewtopic.php?f=4&t=10775&p=381936#p378680

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master