This allows us to differentiate between one or multiple files,
and will also come in use when we want to create a context menu
from the webview later on
We now use representedObjects to store the selected paths.
That should make it possible to reuse the menu's from somewhere else.
Subclasses NSOutlineView to PBQLOutlineView that takes a controller which
should respond to toggleQuickLook:. This is used for our tree walker to
actually show QuickLook when space is pressed.
Nathan Kinsinger