rscope: add outline mode support

Use outline mode for results buffer :
 - enables to collapse and expand trees (hierarchy of calls)
 - enables to navigate more quicky in the result buffer

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
rscope: change defaults

Change default calling hierarchy depth to 4, and key binding to replace
current result buffer by the entry from C-<return> to S-<return> to be
more compliant with xscope default.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>