filechooser: Set date and time after cell is a child of filechooserwidget

filechooser: Set date and time after cell is a child of filechooserwidget

The date/time column relies on the filechooserwidget to format the date
properly.  During bind, the filechoosercell, get the filechooserwidget
ancestor, but now due to changes in the listview, the cell isn't a
child of the filechooserwidget at that point.  Since this is deeply
ingrained into the filechooserwidget, let's keep the same behavior,
but move it to filechoosercell in realize.  Alternatively, we could have
used a signal factory (with the file chooser widget as the user data),
but that would have been a major overhaul.