Improve Solidify/Bevel Modifier cooperation

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).

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -1049,7 +1049,12 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
             col.prop(md, "edge_crease_inner", text="Inner")
             col.prop(md, "edge_crease_outer", text="Outer")
             col.prop(md, "edge_crease_rim", text="Rim")
+            col.label(text="Bevel:")
+            col.prop(md, "bevel_convex")
         else:
+            col.label(text="Bevel:")
+            col.prop(md, "bevel_convex")
+            col.separator()
             col.prop(md, "nonmanifold_merge_threshold")
 
         col = split.column()