Fix bugs with "Expand Clip" (Expand MLT Clips) and add tests

Summary
Expand Clip doesn't work as described here: https://kdenlive.org/en/project/the-library-copy-paste-between-projects/

STEPS TO REPRODUCE

1. Add a library MLT clip with more than 1 clip inside via project bin onto the timeline
2. select the library clip
3. choose Timeline > Current Clip > Expand Clip
4. Nothing happens, but it should extract the clips inside the MLT library.

TASKS

• Fix the bugs mentioned above and below
• Add software tests to prevent regressions

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Related Bugs

• https://bugs.kde.org/show_bug.cgi?id=412481
• https://bugs.kde.org/show_bug.cgi?id=433617
• https://bugs.kde.org/show_bug.cgi?id=378022