singleton(properties, ...new_args) {
			return Class.extend(properties).instantiate(new_args);
		}
Extends this base class with the properties described in
`properties`, instantiates the resulting subclass using
the additional optional arguments passed to this function
and returns the resulting subclassed Class instance.

This function serves as a convenience shortcut for
{@link LuCI.baseclass.extend Class.extend()} and subsequent
`new`.

@memberof LuCI.baseclass

@param {Object<string, *>} properties
An object describing the properties to add to the new
subclass.

@param {...*} [new_args]
Specifies arguments to be passed to the subclass constructor
as-is in order to instantiate the new subclass.

@returns {LuCI.baseclass}
Returns a new LuCI.baseclass instance extended by the given
properties with its prototype set to this base class to
enable inheritance.