XNAT assessment support

The changes add support for assessments. You can browse them and download their resources.
Other changes:
  - tooltips in the tree view
  - scans appear with their scan type instead of their label.
  - exception is caught for invalid URLs. Only for experiments at the moment, it should be handled at another level later.
Are you sure this is necessary on MacOSX? Especially since the method declaration still uses `size_t` as type which is an unsigned integral type.