Curve-path colouring and cache mode settings are used (and can thus be
factored out of) all child types; this moves them into the parent type's
`.__init__()` and adjusts all sub-types match:
- the bulk was moved out of the `Curve.__init__()` including all
previous commentary around cache settings.
- adjust `BarItems` to use a `NoCache` mode and instead use the
`last_step_pen: pg.Pen` and `._pen` inside it's `.pain()` instead of
defining functionally duplicate vars.
- adjust all (transitive) calls to `BarItems` to use the new kwargs
names.
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