Fix #14633: implode from multiple voices accidentals.

[mikekirin]

Resolves: #14633

• 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)

[oktophonie]

This seems to work well, though the colour of the selected accidentals does not change to reflect their move to voice 1:

This remains the case if deselected/reselected, and only corrects itself after a save and reload.