Allow "Pinning" Viewer Node

Sometimes it can be helpful to view how the output of a node group changes when nodes within it are altered. However, when there are multiple levels of node groups, this can be difficult of impossible to achieve quickly and cleanly.

I propose an option to "pin" a viewer node, so that it remains active even when children or parents of the node group it exists within are selected for editing.

To avoid confusion and frustration caused by a forgotten pinned viewer node, I suggest that a conspicuous "clear pinned viewer node" button appear on the top bar of the node editor while a viewer node is pinned.