self.mobjects: send_backwards(), bring_forward() methods.

[ubaldot]

So far there are two methods bring_to_back() and bring_to_front() that send/bring a mobject in position 0 or -1 of the self.mobjects list.

It would be nice to implements two methods that just send/bring a mobject backwards/forwards of just a position.

For example, say that self.mobjects[i] = mob, self.mobjects[i-1] = mob_l and self.mobjects[i+1] = mob_h. Then, mob.send_backwards() would result in self.mobjects[i-1] = mob, self.mobjects[i]=mob_l and self.mobjects[i+1] = mob_h whereas mob.bring_forward() would result in self.mobjects[i-1] = mob_l, self.mobjects[i]=mob_h and self.mobjects[i+1]= mob. The implementation of such methods would be a trivial swap, I think.

Also, the method bring_to_back() could be renamed to send_to_back(). That could be done by adding a DeprecationWarning inviting the user to use the send_to_back() method and by pointing out that that the bring_to_back() method will be soon removed.