Add bulk update mechanism for List/Tree data sources

### Discussed in https://github.com/beeware/toga/discussions/2746

Toga's ListSource and TreeSource APIs have list-like APIs - but they currently only provide `insert()` and `append()`, inserting single rows. There are no "bulk update" APIs, like `extend()`.

Although you can superficially implement `extend(N rows)` with a series of N `append()` calls, there is a potential internal optimisation in only emitting a single notification, rather than N individual notifications.

Toga's list-like Source APIs should allow for "bulk update" in the form of `extend()`, with an optimized notification that can respond to a bulk update.

Start by locating the ListSource and TreeSource APIs and reading how insert() and append() notify observers. Add an extend() operation for bulk rows, with completion shown by a single bulk-update notification rather than one notification per row.

Written by the indexing model from the issue text.