Update Idf.py

Fixes for bugs 0001491, 0001492, 0001493
+ Re-insert workbench box in Complete due to weird behaviour on some systems
+ fix typos or documentation
+ minor change in Vector2D
+ fixes #0001490: Implement a perimeter circle (3 point circle) similar to solidworks in sketcher (mdinger)
+ fixes: #0001475: Implement a 3 point arc similar to solidworks in sketcher (mdinger)
Find Coin3 header files on Fedora.
Merge branch 'patch-1' of https://github.com/Barleyman/FreeCAD_sf_master
Update Idf.py

Fixes from bug 1366 http://www.freecadweb.org/tracker/view.php?id=1366
+ remove plus sign
Draft - enabled utf8 in coin3d and svg - relates to issue #1404