Shared imageset root

[Liontooth]

Currently, all imageset directories must be located within the /annotator directory, or symlinked to it.

Wishlist: Permit a shared directory for all imagesets, say "imagesets", instead of the application location, so that new directories can be added in a different location without having to manually add symlinks for each one.

This can already be done for the parameter files. These can either be placed in the directory /annotator/parameter_files, or in another directory with a symlink like this:

ln -s /mnt/projects/databases parameter_files

Parameter files can then be added to /mnt/projects/databases with no further customization of the annotator.

The request is to allow the same arrangement for imagesets.