External layout improvements

[tristanbob]

Goals of this PR:

1. Update the action sentence to display the position objects will be created at

2. Add a new parameter to pick the objects created by an external layout

TODO:

• [ ] Implement object picking when new parameter is set to "yes"

[4ian]

For objects picking, there is something very important: you'll almost certainly need to have an argument that ask for the group of objects which will be picked. GDevelop can't just "dynamically pick stuff". It has to know, at code generation, what objects will be there. Hence why when you create objects from a text expression, it asks you the group in which the objects must be created (because it means that this group is then picked).