Provide a palette of system colors

[freakboy3742]

What is the problem or limitation you are having?

Toga's style engine has a (largely undocumented) concept of SYSTEM_FONT_SIZE - the default font size for text on the platform. It should also expose a selection of system colors that can be used.

Describe the solution you'd like

Most platforms have a palette of "standard" colors - primary, secondary (, tertiary...) text color, hyperlink color, border color etc. Toga should determine a reasonable subset of "common" system colors, expose those as constants through Pack, and modify color handling so that if a "system" color is requested, a platform appropriate color is used.

This likely means defining constants like toga.colors.SYSTEM_PRIMARY_TEXT that can be mapped to platform-specific shades.

Interestingly, CSS has a specification for this - but they've been deprecated. This seems to be because in the HTML context, the potential for HTML pages to spoof actual system dialogs etc is dangerous - but in our case, that's what we're trying to do. It might make sense to use these constants (or a subset of them), even though they've been formally deprecated from the standard.

Describe alternatives you've considered

Status quo - the system colors will be used if there's no explicit color provided, but it doesn't provide any way to use those colors in a different context.

Additional context

See #2830 for the original request.