Power Monitor (--monitor) fixes

This release sharpens the whatcable --monitor live view in the terminal.

Fixes

Ports show up the moment you plug them in. --monitor used to leave a freshly connected port invisible until macOS got around to publishing its power figures, which can take up to a minute. It now shows the port straight away as reading... and fills in the wattage once macOS reports it. Thanks to @QuantumLibet for the report (#298).
No more permanent "estimating… (0 samples)". The cable resistance line could sit on estimating... forever on ports where a reading can never be produced. It now only appears when there's a real measurement. Thanks to @QuantumLibet (#299).

--monitor now shows column headers and clearer port labels (USB-C 1, MagSafe 3 1, so a MagSafe port is never confused with a USB-C port of the same number).
Desktop Macs label mains power "AC Power" instead of "Charger" in the Power Monitor, since a desktop has no battery.