Transformation Constraint: implement a Mix Mode option.

Allow selecting how the new location/rotation/scale is combined with
the existing transformation. This is most useful for rotation, which
has multiple options, and scale, which previously could only replace.

release/scripts/startup/bl_ui/properties_constraint.py

@@ -761,6 +761,8 @@ class ConstraintButtonsPanel:
         sub.prop(con, "to_min_z" + ext, text="Min")
         sub.prop(con, "to_max_z" + ext, text="Max")
 
+        layout.prop(con, "mix_mode" + ext, text="Mix")
+
         self.space_template(layout, con)
 
     def SHRINKWRAP(self, _context, layout, con):