Support for functional group of nodes

Robotics applications are often divided in reusable modular functional blocks to perform different tasks (person detection, navegation, etc). This can be represented in behavior trees as a group of nodes, also known in pytrees and bt.cpp as sub trees. In the future this subtrees can be added to a comprehensive collection so BT Studio's users can easily access them in their apps.

This support would involve:

* [x] Support for sub trees in tree gardener
* [ ] Visual tools in the frontend to group nodes. This would involve creating, editing, tagging and deleting such groups.
* [x] Backend tools to save the status from the editor with this additional info.