Add option to lock numbering of structural/analytical id

Hi! We have had some struggles lately with objects being re-numbered.
For example, if the walls in a model are called W1, W2, W3.... they will sometimes be called the same thing when the struxml file is opened in FEM-Design again, but sometimes they will be renumbered.

To stop this from happening, we can put "@" before the slabpart name in the struxml. Then the numbering will be "locked".

It is also possible to lock the numbering in the FEM-Design UI, but this setting disappears when exporting to struxml.

We think it should be possible for the user to choose if they want renumbering or not.
When renumbering is good: For example if you want to merge two models, or create a lot of objects and not want to handle potential conflicts between elements with the same name.
When renumbering is bad: When you want to keep track of certain objects.

I'm not sure how this best could be implemented. Some ideas:

• Have a component that locks/unlocks the identifier for any element (bar, slab etc)
• Have a component for "global settings" where this can be changed
• Add input to "save struxml" and "open model" and those components with a toggle for locked/unlocked

This has also been mentioned in case #81

Start by reading related case #81 and inspecting the components for saving struxml, opening models, and handling structural or analytical identifiers. The issue leaves the implementation choice open; done means users can choose whether identifiers are locked or renumbered, while preserving the existing behavior when desired.