Add colors count near reduce pallete

It would be helpful to see the count of colors in the source image when reducing the pallete, and when checking the box to reduce pallete to default it to the same number of colors in the source image.

Start at the palette-reduction controls in the browser interface and trace how the source image and default palette setting are handled. Done means the source image's color count is displayed during reduction and checking the default option uses that same count.

Written by the indexing model from the issue text.