bug#48710: Retaining code highlighting (for defun, defvar) in outline mode

[pauline-galea]

Currently outline mode changes the colour of headings and subheadings.

But I have noticed that elisp structures are also considered as headings.
This results in some code highlighting problems for things like "defun"
and "defvar" because the whole line gets a one colour highlighting in
a way that is similar to a heading face.

It would be better that elisp code structures retain the original code 
highlighting,
but keep the ... at the end of the line to show that the code body is hidden.
The ... could be highligted instead instead.

For more flexibility, it would be beneficial to have a separate keybinding that 
toggles
between showing the contents (body) of a code structure and hiding it, that is 
independent
of headings.