Shape Keys: Add operator to update keys from selected objects

This operator is very similar to the existing "Join as Shapes" operator,
which updates or adds shape key array values with the positions of
objects with the same name, but instead of creating new shape keys,
it just updates existing ones. The new operator is called "Update from
Objects" in the UI.

Internally, some logic was moved to a poll function shared between the
two operators, and a new argument for whether to ensure keys exist was
added to their shared implementation.

Part of #135095.

Pull Request: https://projects.blender.org/blender/blender/pulls/136853

scripts/startup/bl_ui/properties_data_mesh.py

@@ -66,6 +66,7 @@ class MESH_MT_shape_key_context_menu(Menu):
         layout.operator("object.shape_key_mirror", text="Mirror Shape Key (Topology)").use_topology = True
         layout.separator()
         layout.operator("object.join_shapes")
+        layout.operator("object.update_shapes")
         layout.operator("object.shape_key_transfer")
         layout.separator()
         props = layout.operator("object.shape_key_remove", icon='X', text="Delete All Shape Keys")