Single, unified Value node that picks its type from usage

This is blocked on having a way to pick the widgets for a node by clicking its expose dot, which will open a popover for picking the widget type:

Then the user can do this on the Value node (which is basically an Identity node generic over all types) which lets you pick the type to generate with the widget, and thus output with the single Value node.

Related: https://github.com/GraphiteEditor/Graphite/issues/2447