Unable to update select_default property

[JosueBock]

Hello,
At the moment, the "select_default" property can only be changed from its default value (False) if defined when creating the FileChooser:
fc=FileChooser(path="...", filename="...", select_default=True)
but it does not work if a 2 step definition is used:
fc=FileChooser(path="...", filename="...")
fc.select_default=True

I propose a commit to fix this (see ongoing PR)