Write a QCalendar example illustrating the user-supplied plugin mechanism

We should have an example that explains how to write a plugin to implement a custom calendar backend for QCalendar. It should make clear that the backend API is, like the QPA, not subject to the usual compatibility promises.

A candidate example would be a (naïve – don't try to represent Sweden's transition) Julian-Gregorian calendar whose constructor takes a QDate at which to make the transition from Julian to Gregorian. (Unfortunately this can't use QJulianCalendar or QGregorianCalendar – they're not public API – so shall need to duplicate some code from them.)

Writing this example (and testing it works) may well uncover some things in need of fixing in the backend API.