Reordered attribute priming in Node.__init__

[HalfWhitt]

One more minor thing... while testing beeware/toga#2942 against multiple versions of Travertino, I'd mistakenly been ignoring some RuntimeWarnings on main, not just on 0.3.0.

Toga doesn't normally provide an applicator to Node.__init__, but ExampleNode in the tests does. When that happens, styles are applied, meaning the logic for setting hiddenness checks the node's parent, and that attribute wasn't being primed until afterward.

(An alternative would be to alter the property to return getattr(self, "_parent", None), which I've avoided here for consistency with explicit priming. But it could be an option down the line for node attributes, if complexities in Pack and/or other layout engines cause any conflicting needs for the order in which they're available.)

There's no reason root needs to be primed earlier as well, but conceptually I wanted to keep the "up" direction grouped together. Makes a nice "above - here - below" order now.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Makes sense - 👍