SettingsHost class abstract

The port between the panes and a settings object they must not name.

Every member here was read off a call site, not designed. The three panes were typed on the subject's own settings class, and what they actually did with it was: walk the spec, read a switch, write a switch, build one control, and — in one place — draw something no registry entry could express. Five operations, so five members. A sixth would be a guess.

The panes are rebuilt whenever the settings change, so an implementation is an ordinary short-lived value wrapping the current settings and the callback that replaces them. It is not a store and holds no state of its own.

Prior art in this repository: RowLocator, which is how the drag-selection controller asks "which row is at this offset" without knowing the body, and ShellDestinations, which is how the shell receives a menu it cannot name.