Select folder mode

The file chooser will let the user pick an existing folder.

Property definitions

gtk $ GtkFileChooserAction :: select_folder
	# Select folder mode
	#
	# The file chooser will let the user pick an existing folder.
	new select_folder `{ return GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER; `}
lib/gtk/v3_4/gtk_dialogs.nit:195,2--198,70