Open file mode

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

Property definitions

gtk $ GtkFileChooserAction :: open
	# Open file mode
	#
	# The file chooser will only let the user pick an existing file.
	new open `{ return GTK_FILE_CHOOSER_ACTION_OPEN; `}
lib/gtk/v3_4/gtk_dialogs.nit:185,2--188,52