ENH: Add possibility to enable/disable additional paths

Two new properties have been added:
  encodedDirectoryList
  directoryMap

Both property allows to set and retrieve the state of paths: enable or disabled
In the encoded list, disabled path are appended with the string "?disabled=1"

Note: Removed the concept of AbsolutePath. The widget won't transform the
path provided by the user.

Fixes #2256

Co-authored-by: Jean-Christophe Fillion-Robin <JChris.FillionR@kitware.com>