Allow to extend selection by clicked item

[porg]

Reproduction

• You have an active selection (possibly by help of #24)
• You want to extend it by certain single non-consecutive items

Actual: Not possible

Expected: Click with modifier-key down (cmd on Mac, ctrl on Window/Linux)

User Rationale: Select multiple items to move or delete, a quite frequently used function in any playlist editor

[marcrobledo]

Which web browser and OS are you using? This has been implemented since first version.

[marcrobledo]

Wait. I see the uploaded version is not working correctly, my mistake.

This has been already fixed for next version.

[porg]

✅ Tested and worked on: Safari 15.4 on macOS 11.6.5