[internal] Make connecting components together easier and more flexible

[17o2]

Inspired by discussions in #26 and #45 .

• Make it easier to define cable-connector-cable type connections to make creating this kind of layouts easier.
  Currently supported connections are con-cbl-con, con-cbl and cbl-con,
  as well as ferrule-cable and cable-ferrule with ferrule auto-generation.
  Relevant code is here.
• With some clever coding, maybe even arbitrarily long connection sets could be defined, e.g. connector-cable-connector-cable-ferrules
• Allow ferrule-type connectors to have (and show) designators, and include these in the BOM, if visible.
  But probably hide ferrule designators by default.
• Generalize ferrule category to simple(=only one port/pin, easy to auto-generate if designators are not important)

[17o2]

The goal is to generate e.g. this output:

from something like this:

connections:
  - 
    - C1: [1-3]
    - [S1, S2, S3] # Wago's need pre-defined designators so they can be referred to later on
    - C3: [1-3]
  -
    - C2: [1-3]
    - [S1, S2, S3] # join a new cable with the existing Wago's

As the tangles get messier, it might be helpful to set show_name: true for the Wago's,
and for their name to show up next to the respective wires, as it already does when routing to (non-ferrule) connectors.