Add a button to explicitly set the color from the combobox

The currentIndexChanged signal fires all the time whenever anything
changes, like filling the combobox. It's not elegant, but the
explicit set button avoids issues with the foreground color changing
when it's not wanted.

BUG:394705
(cherry picked from commit 0b5f067219e2eb81c8adfcc59c4532259f508b09)