Warn before deleting tracks that contain clips

Yash Bavadiyarequested to merge
xevrion/kdenlive:feat/delete-track-warn into master

Summary

When deleting one or more tracks, show a warning dialog if any of the selected tracks contain clips. The warning displays the total number of clips that will be lost, and lets the user confirm or cancel.

Previously, clips were silently deleted with no indication that the selected tracks were not empty.

How to test

  1. Add clips to a track
  2. Right-click the track header → Delete Track
  3. Select a track that has clips → click OK
  4. Warning dialog should appear with the clip count
  5. Click Cancel → track is NOT deleted
  6. Repeat and click Continue → track is deleted

Fixes #2167 (closed)

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