Replace trait SharedData with DataAccessor

[dhardy]

Revise how the kas-view traits work:

• Remove traits SharedData, ListData, MatrixData
• Add trait DataAccessor<Index> where Index is usize for ListView or (usize, usize) for MatrixView; queries are simplified relative to List/MatrixData while supporting async results
• Tweak trait Driver: change generic parameters, rename fn on_messages → handle_messages and change its parameter data to &Item.
• Replace widget FilterBoxList with FilterBoxListView, a similar but more constrained widget over the new DataAccessor trait.
• Remove widget FilterList and UnsafeFilteredList adaptor.

Unfinished in this PR: scroll bars require the data len to calculate their length properly; for this reason primarily we require that fn len return a fixed Index value. We should allow some flexibility here, maybe like Iterator::size_hint or maybe some other way. For now we just document the limitation.

Another limitation is that fn DataAccessor::item must return a reference. It could return an Option<Cow<Self::Item>> instead to allow in-place generation of values, or maybe something like impl Option<Borrow<Self::Item>>. Is the extra API complexity worth it? Note also that the API usage expects that fn item is fast.

Yet another limitation: FilterBoxListView accepts &() input data since its filter is passed as input data to the data accessor. Ideally both filter and this widget's input data would be passed into the accessor, but our input data model is too limited to allow this (#488).

Also:

• Rename fns push_async, push_async_erased, push_spawn to send_* since they take id as input like fn send unlike fn push.
• autoimpl trait Scrollable for MapAny