Currently changing target directory from GUI doesn't always work. If
user chooses directory from drop-down list of GtkFileChooserButton,
'current-folder-changed' signal isn't emitted and configuration isn't
updated.

Instead of using 'current-folder-changed' signal from GtkFileChooser
interface, use 'file-set' signal from GtkFileChooserButton.

Furthermore get_currect_folder() doesn't correspond to the selected
folder, but to the folder containing selected folder. Therefore it is
changed to get_filename(). For more info see manual for
gtk_file_chooser_get_current_folder().