Directory tree support for <important-file> shortcuts

It is now possible to define directory trees for the <important-file>
shortcuts. Simply add \ or / into the 'title' attribute value of the
<important-file> property and let npackd automatically make/remove the
directory tree structure at the install/uninstall phase.

This integrates a non-destructive cleanup process. Which means that the
directory trees containing the shortcuts will be automatically removed only
if empty.