[FEM] fix body heat source constraint

- as the name implies, it must be possible to set the heat to different bodies
- for this a selection dialog is necessary
- check that heat is not zero

src/Mod/Fem/femobjects/constraint_bodyheatsource.py

@@ -40,8 +40,6 @@ class ConstraintBodyHeatSource(base_fempythonobject.BaseFemPythonObject):
     def __init__(self, obj):
         super(ConstraintBodyHeatSource, self).__init__(obj)
         self.add_properties(obj)
-        # the constraint does not take any references, thus hide this property
-        obj.setEditorMode("References", 2)
 
     def onDocumentRestored(self, obj):
         self.add_properties(obj)