Implement group actions

[juzzlin]

Make it possible to set color, text color, delete etc for a node group.

[juzzlin]

This is a bigger fundamental change to the way group selection works.

Now it's possible to:

• Move group by move handle
• Set text color for a node group
• Set color for a node group
• Set attached image for a node group
• Delete multiple nodes
• Set colors for a group also via node handle

Implemented in 0f7fc8ff439f075a9d2b608d946ff488998c1685