ENH: Create editable directory list

Create utility widget class DirectoryListWidget, which provides a
standard interface of supplemental buttons to support a user editable
qSlicerDirectoryListView. (Surprisingly, no such class previously
exists; existing occurrences of this pattern reimplement the
supplemental UI in the parent, for each occurrence!)

This will allow us to trivially create multiple instances of editable
lists.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23282 3bd1e089-480b-0410-8dfb-8563597acbee