Manual frame extraction fails when videos are not copied to the project (as mentionned in issue #2502), as the VideoSelectionWidget resolves symlinks automatically. Thus napari-deeplabcut gets a path to the video in its true location instead of the symlink, and does not extract the frames to the correct folder.

This PR adds a method to check that the video is in the project folder, and gets the path to the symlink again if the path was resolved. It raises an error if the video cannot be found in the project's videos/ (e.g., if the user selected a video for manual frame extraction which was not added to the project).