Support for functional group of nodes

[OscarMrZ]

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:

• 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.
• Backend tools to save the status from the editor with this additional info.