Clock: Independent face for clock overlay display.

Clock: Independent face for clock overlay display.

William Henney writes:

     If a heading has tags, then that heading's time summary
     overlay (shown by C-c C-x C-d) is displayed using the
     org-tag face. This looks a bit strange. Is it intended? It
     is especially noticeable in my case, since I have my tags
     shown in a smaller font.

This is cause by the fact that we use the `secondary-selection' face
here which only highlights but inherits the rest from what is below.
The idea was that these clock times should have the same color as the
headline they belong to, but this idea is defeated when a line has tags
with a very different face.

This commit introduces a separate face for this purpose, so that users
can customize it.