Fix T75032: New complex solidify algorithm handles poorly merging threshold of small geometry details.

* Implemented the algortihm that would merge vertices to the weighted
  center between them.
* Exposed the merge threshold to the user.

The new default tolerance is 0.0001 (versionning code ensures that
previous default value remains in use to avoid any change in existing
files).

Review and minor changes/cleanups from Bastien Montagne (@mont29).

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -1049,6 +1049,8 @@ 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")
+        else:
+            col.prop(md, "nonmanifold_merge_threshold")
 
         col = split.column()