Fix #15726 (Spacers): Some elements are lost when changing time signature

[pacebes]

Resolves: #15726 (Spacers)

This PR keeps as much Spacers as possible when changing time signature

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[pacebes]

Two Scores to test the PR with duplicate Scores to facilitate testing are provided: Score_15726_spacers_tests.zip