Add Row and Column as subclasses or shorthands 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.

[mhsmith]

This also helps people who are taking advantage of the fact that "row" is the default direction to save typing. toga.Row would save even more typing, while also being more explicit.