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: