export-to-folder: Fix setting target directory
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().