Add bulk update mechanism for List/Tree data sources

[freakboy3742]

Discussed in #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.