Addresses issue #2279.

When manually extracting outlier frames from a video (not located in the {project}/videos/ folder), two issues were occurring:

• the video was not added to the project
• the path to the video was given to napari-deeplabcut, instead of the path to alias (or copied video) created in the {project}/videos/ folder.

There napari-deeplabcut tried to extract the frames to a labeled-images folder located in the parent folder of the analyzed video (which logically did not exist). This pull request fixes these issues.