Auto-update trace dialog when toggling traces

When toggling trace on a definition, automatically update the
trace dialog if it is open already. Idea by LuÃ­s Oliveira.

* contrib/slime-trace-dialog.el (slime-trace-dialog--get-buffer): split
into reusable functions.
(slime-trace-dialog--buffer-name): new function
(slime-trace-dialog--live-dialog): new function
(slime-trace-dialog--ensure-buffer): new function
(slime-trace-dialog--update-existing-dialog): new function for
added to `slime-trace-dialog-after-toggle-hook' by default.
(slime-trace-dialog): Use `slime-trace-dialog--ensure-buffer'.

* doc/slime.texi (SLIME Trace Dialog): Correct typos.