Make Accessors Optional for `ListSource` and `TableSource`

### What is the problem or limitation you are having?

Currently the `accessors` argument is required when creating a `ListSource` or `TreeSource` but it is only *used* when converting sequential data (like lists and tuples) to `Row` or `Node` objects. For data sources where rows are completely constructed from mappings it is redundant. The `accessors` are also a private data item so there is no public way for a user of the data source to ask how sequential data should be supplied.

### Describe the solution you'd like

Make the `accessors` argument optional, and rather than raising an error when constructing the data source, ~instead only error if it is absent and the user actually supplies a sequential data value for a row~ if the accessors are not supplied then treat sequences as opaque objects and pass them as `{'value': data}` to the Row's `__init__` method (perhaps after converting them to tuple so they are immutable).

Also make it a public read-only attribute (perhaps as `accessor_order` or something similar).

### Describe alternatives you've considered

Keeping the status-quo. It works. This is just streamlining the API.

### Additional context

This came out of this comment: https://github.com/beeware/toga/pull/4042/changes#r2680642184

Start at the ListSource and TableSource constructors and follow how accessors are used when sequential data is converted to Row or Node objects. Check the related data-source tests, if present, and confirm that mapping-based rows work without accessors, sequences are passed as opaque values, and the accessor order is publicly readable.

Written by the indexing model from the issue text.