Add a demo() function for illustrating color palettes?

[hneth]

Given that usecol() and newpal() are powerful and versatile functions for creating new color palettes, it would make sense to include a demo() function that applies a given palette to typical visualizations.

For instance, see the demoplot() function of the colorspace package.

PS: By contrast, the show_col() function of scales is similar to seecol().

[hneth]

Today's release of unikn version 0.6.0 contains an initial demopal() function. But as this can still be extended in various ways (e.g., by adding plot types, showing a color legend, or providing better support for overlapping/transparent color areas) I'll keep this issue open for now.