Allow setting src name for mapping tree elements

[liach]

It's weird that people can set the dst name but not the src ones.

[sfPlayer1]

It is just not implemented yet, the uses in indices makes it a little trickier than dst name handling.

[liach]

main use case is that in outer class name propagation, i want to have a generic setName like getName in case the updated namespace turns out to be the pivot namespace.