Gui: clean-up Selection API

Replace the int of the 'resolve' argument of several functions with a proper enum class.
* This avoids the inconsistencies in client code where often true/false is passed when an int is expected
* This avoids the use of magic numbers like 0, 1, 2 or the undocumented 3

src/App/FreeCADInit.py

@@ -892,10 +892,7 @@ App.Units.KinematicViscosity           = App.Units.Unit(2,0,-1)
 App.Units.VacuumPermittivity           = App.Units.Unit(-3,-1,4,2)
 
 # Add an enum for the different unit schemes
-if sys.version_info.major < 3:
-    IntEnum = object
-else:
-    from enum import IntEnum
+from enum import IntEnum
 
 # The values must match with that of the
 # C++ enum class UnitSystem