Blender Git Loki

List behavior for the mesh-combine node.

This is primarily a python implementation for UI convenience.
It makes the list of input sockets behave dynamically for typical list operations:
* adding an element at the end
* inserting an element
* removing an element

Each of these keeps the list compact and shifts remaining sockets, so users don't
have to move each individual connection every time.

Internally it still uses the simple binary combine operation, adding as many
nodes as necessary.