Blender Git Loki

Group Nodes made functional: each group now can be re-used (instanced)
with SHIFT+G. This works as well for local groups as library-linked
groups.

Also fixed that group nodes were copying internal data to the outside,
which made it impossible to use the socket-buttons to set individual
values for each group-instance.

Library-linked groups are prevented from editing. But, try to open a
group and it will give a request for 'make local'. The make local rule
is identical to other library data in blender, meaning:
- if all users of the library data are local -> the library data is
flagged 'local', and if needed a unique name is made
- if there's mixed users (local and from other library data) it makes
a full copy, and assigns this copy to all local users.