Show every monitor on a dock

When two monitors were connected through one dock, WhatCable showed only one of them. macOS reports each monitor on a dock as a separate display that shares a single port, and WhatCable was collapsing them into one. It now tells them apart, so every monitor shows up: in the menu bar popover, the Display Diagnostics window, the CLI, and the widget. The same fix covers three or more monitors on one dock.

Thanks to @jsumner for the clear report.