Allow for a library of configurable CPUs

[tomcl]

Is your feature request related to a problem? Please describe.
Include a library that allows users to directly configure different types of CPUs, similar to what most simulation software offers.

Additional context
This is complicated.

1. There is nothing to stop adding CPUs as library components, with hidden design sheets, except that all the design sheets get mixed up with user design sheets.
2. It would be good to have a proper library of components but that requires library components to be configurable with constant parameters. (see the New Issie custom components with program-defined internal hardware content #199).
3. Maybe what we need is a better way to include library sheets (some hidden). E.g. a function to export a design as a library component in a single file with .lib suffix which packages all design sheets together and marks sheets on load so that the interior ones are hidden and the top one cannot be edited (but appears under a new "Library" tag in catalog. This would be useful, although without the ability to customise such complex components it is of limited use.

Need some creative thinking about how to do customisation nicely within the existing nice and simple design sheet system.

The real challenge here is that adding complexity for advanced users may make errors or confusion more likely for novice users.