- move more active properties into their collections:

scene.active_keying_set --> scene.keying_sets.active
  ...same for active_uv_texture. active_vertex_color, active_keyconfig, 

- move mesh.add_uv_layer() and mesh.add_vertex_color() into their collections
  also have them return the newly created layer and dont set the layer active.

  uvtex = mesh.uv_layers.new(name)
  vcol = mesh.vertex_colors.new(name)

release/scripts/op/object.py

@@ -427,13 +427,13 @@ class JoinUVs(bpy.types.Operator):
         if is_editmode:
             bpy.ops.object.mode_set(mode='OBJECT', toggle=False)
 
-        if not mesh.active_uv_texture:
+        if not mesh.uv_textures:
             self.report({'WARNING'}, "Object: %s, Mesh: '%s' has no UVs\n" % (obj.name, mesh.name))
         else:
             len_faces = len(mesh.faces)
 
             uv_array = array.array('f', [0.0] * 8) * len_faces # seems to be the fastest way to create an array
-            mesh.active_uv_texture.data.foreach_get("uv_raw", uv_array)
+            mesh.uv_textures.active.data.foreach_get("uv_raw", uv_array)
 
             objects = context.selected_editable_objects[:]
 
@@ -451,10 +451,9 @@ class JoinUVs(bpy.types.Operator):
                             if len(mesh_other.faces) != len_faces:
                                 self.report({'WARNING'}, "Object: %s, Mesh: '%s' has %d faces, expected %d\n" % (obj_other.name, mesh_other.name, len(mesh_other.faces), len_faces))
                             else:
-                                uv_other = mesh_other.active_uv_texture
+                                uv_other = mesh_other.uv_textures.active
                                 if not uv_other:
-                                    mesh_other.add_uv_texture() # should return the texture it adds
-                                    uv_other = mesh_other.active_uv_texture
+                                    uv_other = mesh_other.uv_textures.new() # should return the texture it adds
 
                                 # finally do the copy
                                 uv_other.data.foreach_set("uv_raw", uv_array)