[CO] Improve dxf import

[EmileDvs]

Hello,

We would like to improve dxf import of holes and slots on several aspects:

1. for all dxf import widgets, add matplotlib toolbar in particular to ease zoom and translation
2. for all dxf import widgets, add tolerance to dxf import to prevent from orphans segments/arcs that would be connected if tolerance was lower
3. specifically for hole dxf import widget, enable to load a pre saved hole geometry (as .json format) and fill the list of surfaces. This would enable to modify a previous dxf import in terms of magnetization direction and scaling factor.

Please let me know if you have some remarks/questions,

Emile

[EmileDvs]

Issue solved with PR #453