Where QET stores cable data: a short agreement before more cable tools are built

[ispyisail]

Several people are building cable tools at the same time: Kellermorph's cable manager (#1267), the cable catalogue from #936, and the cable symbols users already draw. If each tool stores cables its own way, they won't work together and no report can read them.

This proposes only where cable data is stored. No code, and nothing about how a cable is drawn or picked.

1. Cable name goes in the wire's existing cable field, not the wire label. Every QET version keeps it on save. (Its edit box is switched off in the properties dialog today; turning it on is a small first step.)
2. Core colour goes in the wire's colour text, as IEC 60757 codes: BN, BU, BK, GNYE…
3. Cable designation letter is W (-W1), as the shipped cable symbols already use.
4. Cable settings do not copy across a potential. Core 3 on one side of a terminal is not core 3 on the other.
5. The project carries its own cable data. A cable type picked from a CSV or catalogue is copied into the project, so it opens the same on another PC.
6. Reports read the project database, not the drawing.

Left open: how a cable is drawn (line across wires, marker, symbol), catalogue format (CSV, SQLite), shields, twisted pairs, spare cores.

Questions

• @scorpio810: yes or no on each rule? Agreement only; no cable code before 0.200.2, as you asked.
• @Kellermorph, @arummler: does your work fit? Which rule would get in your way?

This replaces #956.