Negated enable for multiplexer

[cosineblast]

This is an in-progress PR, consisting of an implementation for negated enables for the multiplexer, and eventually for the demultiplexer as well.

As of now, the feature modifies the enable attribute for the multiplexer to add an invert option, which inverts the input for the enable in the component, as well as adding the little indication circle. This new invert option for the enable attribute takes the value id of 'inverted' in the XML.

So, are the designs for the invert enable OK? If that's the case, I will then work on the demultiplexer as well.