Order of sub elements is not conserved if weights are the same

Problem/Motivation

Sorting of slots does not conserve the insertion order when weights are all the same.

Steps to reproduce

Add a custom element with multiple sub-elements in a way that all child elements have the same weights.
When processed, the slots get out of initial order.

Proposed resolution

Alter the weights of slots by adding a small increasing value to each weight respectively.

Remaining tasks

User interface changes

API changes

Data model changes