Align/Justify aliases don't work with __contains__

[HalfWhitt]

Describe the bug

The aliases added in #3113 work for getting and setting, including with subscripts, but they raise an error when using in. It's probably a pretty simple fix, but I haven't dug into it yet. I wonder if there might be other edge cases too...

Steps to reproduce

from toga.style.pack import Pack
style = Pack(vertical_align_items="start")
"vertical_align_items" in style

Expected behavior

in should return True or False, indicating whether the property has been set or not.

Screenshots

No response

Environment

• Toga: main branch

Logs

Traceback (most recent call last):
  File "<python-input-13>", line 1, in <module>
    "vertical_align_items" in style
  File "/Users/charles/toga_dev/toga/travertino/src/travertino/declaration.py", line 408, in __contains__
    getattr(self.__class__, name).is_set_on(self)
    ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
AttributeError: type object 'Pack' has no attribute 'vertical_align_items'

Additional context

I'm starting to wonder if it might be worth adding a notion of aliases to Travertino itself — it could make this process less ad hoc, and easier to predict. However, I'm not sure if there's a feasible way to make it sufficiently flexible to be useful.

[freakboy3742]

Good catch; definitely something we'll need to look into.

As for adding aliases to Travertino... I'm not expecting that we're going to need a lot of them, so I guess it comes down to whether the mechanics of an alias is easier to coordinate in a "meta" layer. For now, I'm inclined to put that idea on the back burner, and if we find find that there's value in the abstraction (which could include fixing this bug in a consistent way), then we can revisit the idea.

[HalfWhitt]

The same problem exists for the padding/margin aliases, as well as alignment/align_items.