Bug 448716

When working with large projects, at least for my workflow it would be great to be able to delete clips from the disk (i.e. not only removing them from the project) in order to free up space.

Also it would be nice to be able to "manage" proxy clips on disk:
- either by being able to delete "unreferenced" proxy clips from disk (i.e. proxy clips for clips which are not part of the project anymore)
- or by being able to delete proxy clips, when clips are deleted (as described above).

Since proxy clip file names consist only of a (presumably) file hash, there is no easy way to remove them, when corresponding clips are deleted.

Currently, I
- remove unused clips from the project bin
- use a file manager to compare the list of the remaining clips in the project bin to the clips in my Clip Material folder
- delete unused clips in the file manager
If I have to free more disk space, I additionally
- remove all proxy clips
- re-open the project and re-generate all proxy clips.