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Nodes: Display vector values expanded, except when specified as compact.

Change the default drawing method of vector sockets to use a column layout.
This allows more interactive use of nodes and avoids the one or two clicks
needed to edit vector sockets in the old default component layout.

Developers may still use the component layout by flaging the socket with the
`SOCK_COMPACT` socket flag. For now, we choose to draw the SSS scale vector
of the Principled and SSS nodes in a compact form because they don't require
much interactivity.

Reviewers: brecht, JacquesLucke

Differential Revision: https://developer.blender.org/D5458