Gets a property with a given name.
@throws NS_ERROR_FAILURE if a property with that name doesn’t exist.
@throws NS_ERROR_NO_INTERFACE if the found property fails to QI to the
given iid.
Sets a property with a given name to a given value.
Returns true if the property with the given name exists.
Undefines a property.
@throws NS_ERROR_FAILURE if a property with that name doesn’t
already exist.
Returns an array of the keys.