Fix select_default property assignment and interaction

[JosueBock]

1. Allow the select_default property to be assigned in a 2 steps process:
   fc=FileChooser(path="...", filename="...")
   fc.select_default=True
   So far, that was not possible (no err message, but no update of the property).

2. Fix the interaction, by allowing the observe method on "value".
   Sample code (jupyter) showing the change of behaviour:

from ipyfilechooser import FileChooser
import ipywidgets as widgets
from ipywidgets import interact

fc = FileChooser('/home')

def f(x):
    return x

interact(f, x=fc)