[Bf-python] Activation of the material slots

[Jean Montambeault]

When separating > By loose parts, each object inherit of all the 
materials of the original object. This means, quite often that it is 
burdened with materials that are not assigned to any face and yet are 
persistent. Cleaning this up implies a long workaround in Python while 
making it possible to activate them, e.g. in the manner that we already 
activate vertex groups, for testing and eventual removing would simplify 
everything.

Thank you.

J.