Bug 369061

When trying to expand a library/MLT clip in the timeline, and one or more producers cannot be loaded, Kdenlive hangs and has to be terminated manually. Loading of producers may fail because the library/MLT clip contains relative(!) paths and Kdenlive may have been started from a different dir when expanding compared to when the library/MLT clip was created.

So we basically have:
1. Kdenlive hangs when producer cannot be loaded. At least, Kdenlive should give a warning and then continue without hang. It would probably be best if the producer would be created anyway and expansion continued. Upon save, then reloading the project, users would be able to fix these producers.

2. library clips should be created using absolute producer resource paths in order to avoid expansion failing when Kdenlive is started from different dirs.

Reproducible: Always