Default directory for Image Viewer

##### Image Analytics version / Orange version

N/A

##### Expected behavior

Image Viewer should be able to show images from some given directory that could be set in the widget. The column with the widget name would contain only the file name or relative directory.

##### Actual behavior

Image Viewer works for absolute paths. For file names only, or for relative paths, one must (manually) run Orange from corresponding directory.

Start by locating the Image Viewer widget and inspect how it resolves absolute, relative, and filename-only paths. Done means the widget accepts a configurable base directory and displays images using filenames or relative paths without requiring Orange to run from that directory.

Written by the indexing model from the issue text.