Add Row and Column subclasses of Box

[mhsmith]

Rows and columns are used more than any other component, and often don't have any style properties other than direction, so this would save a lot of typing.

I think it would also make the structure of the UI easier to visualise from the code if the direction was a bit more prominent rather than buried in a property. Compare:

toga.Box(style=Pack(direction="column"), children=[...])

with:

toga.Column(children=[...])

[freakboy3742]

The one notable argument against Row/Column classes is that it merges style with content - in principle, with a full CSS engine, it should be possible to take a single composition of boxes, and turn it into a near arbitrary layout.

There's also the question of what happens when someone writes toga.Column(style=Pack(direction="row"), children=...).

However, pragmatically, we don't have a full CSS engine yet; and most users aren't thinking about their app from a "full separation" lens, so I guess including Row/Column as a feature, and documenting the "problems for advanced users" is a reasonable compromise.

[mhsmith]

Merging style with content is indeed what I was proposing in #3011. In that model, Column and Row would override the direction property to make it read-only, so toga.Column(style=Pack(direction="row")) would cause an immediate exception.

Alternatively, I guess Column and Row could just be shorthand for "Box with a default direction" rather than full subclasses.

Let's first decide how to proceed in #3011, and then come back to this.