[Draft] Draft Edit - various fixes (fixes #3537)

https://forum.freecadweb.org/viewtopic.php?f=3&t=29743&start=70
To solve issue with Hidpi display  (fixes #3537), but also to improve node selection, I used some of @looo code from pivy graphics into the tool. 
So:
- i got rid of the action function
- i separated selection and editing callbacks
- i got the mouse position from the callback event and not from FreeCADGui.ActiveDocument.ActiveView.getCursorPos()
- i rewrote the node selection mechanism using soraypickaction instead of getObjectInfo() and getObjectsInfo()
- i tried to put a bit of order in all the mess i made and try to document it.
- also fixed some of the preview code.
- i'm really really sorry for not being able to split the commit in multiple one, but the changes are huge and i was just able to separate minor things...