Adds a slider to solidify which allows the user to add bevel weight on the outside
or remove bevel weight from the inside.
Also includes a very small improvment for working with subsurface modifier where
the rim edge in complex solidify will now also have a chance to get a crease if
there is only two adjacent edges.
Differential Revision: https://developer.blender.org/D7334
Reviewing and minor cleanups: Bastien Montagne (@mont29).
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