Integration of Qt backend with system themes

[johnzhou721]

What is the problem or limitation you are having?

Qt backend currently cannot integrate with system themes on Linux.

Describe the solution you'd like

I, with reviews by Russ, has created, integrated into BeeWare, and gotten accepted https://github.com/beeware/system-pyside6 -- a package that solves this problem. The readme and the issue referenced by the first PR on that repo has additional information about why using the system PySide6 runtime is needed.

To fix:

1. Add in
   https://github.com/beeware/toga/blob/main/qt/pyproject.toml#L62-L68
   a system extra that installs system-pyside6==0.1.0 instead of the regular PySide6 packages.
2. In https://github.com/beeware/toga/blob/main/docs/en/reference/platforms/linux/qt.md at the bottom, add that toga-qt[system-pyside6] can be used to use the system PySide6 runtime installed through the system packages that provides provides PySide6; explain that since PySide6 bindings are static, they are specific to one version of Qt and thereforre PySide6s needs to be provided through the system package manager to use the system Qt runtime which will integrate with system-provided themes.
3. In https://github.com/beeware/toga/blob/main/docs/en/reference/platforms/linux/qt-prerequisites.md, explain that to integrate with system pyside6, installing PySide6 in the system (which auto-installs Qt) is additionally needed; the PySide6 system packages, with minimum distro versions, may be found at the README of https://github.com/beeware/system-pyside6.

Describe alternatives you've considered

There really isn't any after a lot of discussion.

Additional context

No response

[HalfWhitt]

I know you and Russ have been discussing this a lot, and if I'm not mistaken, this is settled as the way to go, right? In a situation like this, where you know exactly what needs to be done and there's no need to discuss further on an issue, it's fine to jump straight to a PR. (Though now that this issue exists, you can of course mark the PR as closing it.)

[johnzhou721]

@HalfWhitt Yes, this is the way to go; however this is a bit down my personal todo-list, and I figured it'd be a good first issue that has a large impact.

[HalfWhitt]

Ah, fair enough. I'll add the tag.

[johnzhou721]

Working on this myself since this is a blocker for more natively-behaving statusicons for Qt.

Refs #4027.