Fix #20962: Keep selection when deleting, changing note/rest lengths

[XiaoMigros]

Resolves: #20962
Resolves #13469

When changing the duration of several selected notes or rests at once, those notes or rests remain selected.

• 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)

[ghost]

Behaviour not perfect in a case where multiple notes or rests are converted to tuplets, but this is a wider problem with tuplet generation which should be solved separately and generally. In other cases it works as expected (when multiple notes are changed, they are list-selected).

[bkunda]

🟢 Re-tested on macOS. Approved.