filechooserbutton: Update the button's state only on programmatic changes to the filechooser's state

The idea is that the button will only update its state of the selection and current folder
when changes to those are done either by the calling program (with the filechooser's API)
or when the user actually confirms and dismisses the underlying GtkFileChooserDialog.

If the user makes changes to the dialog but has not dismissed it yet, those changes
will not be reflected in the button (as one would expect).

This commit also makes sure the current-folder-changed and selection-changed signals
are emitted at the right times.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>