Compositor:
Viewer Nodes inside of Groups now work too. To not frustrate interactive
speed, the following rules apply:
- Making a Group editable (or closing) doesn't signal recalculation of
composite
- clicking on an Input socket always checks changes and calulcates
- When there are Viewers inside an edited Group, Viewers in the main tree
are not executed.
Also added: a "hide unused sockets" icon in the header of Viewer nodes.
This allows cleanup of Groups, to prevent these sockets get reveiled.