Fix WinForms button color rendering

[johnzhou721]

Fixes #4342.

This PR fixes the button rendering issue by taking the following approach:

• Storing native colors for defaults and converting on-demand instead, so system colors work.
• When resetting background color and when we're resetting to a predefined color for the widget, reset to default and set UseVisualStyleBackColor on the widget if it exists on the widget.
• All the bookkeeping changes from the above.

Draft for now to run CI.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by:

[johnzhou721]

Docs linting should be intermittent; this is ready for review, but I would appreciate a formal approval for this only after #4434 [edit: is merged], since the two conflicts. Feel free to do a review of the code and point out changes or comment without approval, though.

[johnzhou721]

This has adapted to #4434, and is ready for final review; reviewers should manually test both #4434's behavior and the behavior documented in the issue of the PR itself (i.e. run toga-demo and verify that the internal appearance of the button is consistent).