One piece idler for Bed Carriage for Tevo Tarantula

This part takes place of 3 parts of this thing : https://www.thingiverse.com/thing:2944122
2x "side_panel_2" and 1x "idler_back", with correction of the bad holes.

Easier to print and to put together, less screws, etc.

Also publishing freecad file if somebody needs to refine.