Use combining faces for highlighting.

Use combining faces for highlighting.

This allows people to define their own faces, yet have highlighting
based on readedness (etc.) to take place.

* gnus-salt.el (gnus-tree-highlight-node): Ditto.

* gnus-sum.el (gnus-summary-highlight-line): Ditto.

* gnus-group.el (gnus-group-highlight-line): Use combining faces.

* gnus-compat.el: Define compat function `add-face' from Wolfgang
Jenkner.

* gnus-util.el
(gnus-put-text-property-excluding-characters-with-faces): Removed.