Draft: gui_selectplane cleanup
Draft: gui_selectplane cleanup

Small spacing fixes like imports in separate lines
for more clarity, and the position of the license.

Added many docstrings to clarify the methods, and spaces
after commas in order to comply with Python PEP8 style.
Also break many lines so that they are shorter than 80 characters.

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