Node groups UI

[Durman]

Addressed problem description

#3319
In this PR I'm going to add Group node and all functionality responsible for drawing its user interface. Adding group nodes is too big functionality I would be able to add it in one PR.

Preflight checklist

• Add group menu
• Add separate trees for groups
• Handling sockets creation
• Add tree description
• Add group in and group out nodes into menu
• Opertaor for creating group from selected nodes
• Operator for ungrouping
• Add the check if selection can be converted into a group
• Take into account frames during grouping / ungrouping (rerouters ?)
• Add import error
  _
• Code changes complete.
• Code documentation complete.
• Manual testing done.
• Ready for merge.

[Durman]

[Durman]

It turned out that coding grouping and ungrouping operators are harder than I thought. Even performance is the issue. I think main problem is array like structure of tree nodes and tree links. Also that straight references should not be keepped. I think I have to create module which will convert tree in more convenient data structure what make much easier manypulate with it.

[Durman]

Funny thing is that most nodes have Node as first base class but in this case poll method does not work.

class VectorsOutNode(bpy.types.Node, SverchCustomTreeNode):