Don't show "Convert to Subgraph" button when selection is empty group or single subgraph node

[christian-byrne]

Don't include the "Convert to Subgraph" item in the selection toolbox under conditions where it doesn't make sense, such as:

• The selection only includes groups (no other types of selectable items)
• The selection only includes a single node and it is a SubgraphNode
• Potentially others?

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[webfiltered]

Was going to mention reroutes, too.. but it turns out I set it to only show when you right click on an actual node (which always includes the node in the selection).

[christian-byrne]

In #4634, I made it not show selection toolbox if only reroutes are selected.

[webfiltered]

Oh that's awseome. I didn't even notice; I just stopped being annoyed by it. =)