Custom help text for this application. Set this to provide custom help
text. Use {ARG} as a placeholder for the program name. If empty, the default auto-generated help from OptionEntry definitions will be
used.
Returns a configured OptionContext for this application. Subclasses
must implement this to create the context with base_options as main entries and add their local options in an OptionGroup.
Returns custom help text for this application. If 'help' property is
set, outputs it (with {ARG} replaced) followed by auto-generated options from GLib's OptionContext.