Provide a palette of system colors

### 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](https://drafts.csswg.org/css-color/#css-system-colors) - 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.

Start by reviewing issue #2830, the existing SYSTEM_FONT_SIZE handling, Pack, and toga.colors. Compare the proposed subset with the CSS system-colors specification and inspect platform color handling. Done means an agreed set of system-color constants is exposed and resolves to appropriate platform colors when requested.

Written by the indexing model from the issue text.