bug#48823: outline-minor-mode and comment outline headings

[feodor-sverdlov]

I have noticed that comment type sub-headings get highlighted with a different colour only when 

"outline-minor-mode-highlight" is set to "override".  But then the code headings do not keep the

code highlighting.  Then if you put "outline-minor-mode-highlight" to "append", so that that  code

retains the code highlighting, the sub-headings levels will use a single highlighting colour.

This outline highlighting implementation in "outline.el" has not been very well thought through.

Code highlighting should be retained, whilst comment type highlighting should use a different colour

according to the heading level.