bug#57813: Icon images are non-functional

[Juri Linkov]

> This patch distinguishes two cases of using buttons in outline-minor-mode:
>
> 1. when buttons can be inserted to a read-only buffer,
>    then it's possible to move point to the inserted button
>    and use it by keyboard;
>
> 2. when the buffer is editable and should not be modified
>    to insert buttons, when the only way to display buttons
>    is with a before-string overlay.  The disadvantage is that
>    buttons can be used only by clicking mouse.
>
> A new variable 'outline-read-only' is added here to allow
> setting it explicitly in buffers that can be modified
> to insert buttons like the Help buffer:

Pushed with the new variable named outline-minor-mode-insert-buttons.