New Issie custom components with program-defined internal hardware content

[tomcl]

Is your feature request related to a problem? Please describe.
The list of Issie built-in components is not large enough to make all design work easy. We need an easy way to add complex and simple library components.

Describe the solution you'd like
Add a new component type which is similar to custom components, but the internal logic defining function is generated programmatically from the catalog dialog, and kept as a field of the component

Describe alternatives you've considered
See #147 for a much more complex feature that covers some of the use cases. #147 and this feature are partly orthogonal but have some overlap.

Additional context
This feature is needed in any case for Verilog component implementation - where a programmatically generated Issie design with the same functionality as the verilog is the easiest way to make verilog components simulate correctly.