Allow a hook to alter the list of CSS files parsed by Color module

Description of the need

Color module provides preset color schemes. color.inc provides the schemes, the default colors which are swapped for scheme colors, but also a list of which CSS files are searched for these default colors. The CSS files in that list must also be ones declared in the theme's info file.

It would probably be useful if contrib modules which style their output, could also expose their output to color. So if a module created a new front page slider for example, it would be good if changing theme color schemes would change the slider colors as well, to match the rest of the display.

Proposed solution

A new hook, that lists CSS file locations to expose to Color module.

Alternatives that have been considered

None

Additional information

Note that this would (only) be helpful once we allow color info to be config, in https://github.com/backdrop/backdrop-issues/issues/2554

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes the ability to expose module color styles to Color modules schemes.

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Advocate @docwilmot