removing an image sequence from the project can remove the wrong one

[Alex Merry]

OpenShot 1.3.0

OpenShot does not appear to be able to distinguish between image sequences with the same pattern; trying to remove an image sequence from the project (by right-clicking it and selecting "Remove from project" in the Project Files list) simply removes the first image sequence with that pattern.

Steps to reproduce:

1. Create a new OpenShot project
2. Add an image sequence, say with the pattern P%d.jpg
3. Add another image sequnce based on a different directory, but with the same pattern
4. Right-click the second image sequence and choose Remove from Project

The first image sequence will be removed, instead of the one that was selected.