New `slime-trace-dialog-copy-down-to-repl' command.

* contrib/slime-trace-dialog.el (slime-trace-dialog-mode-map): Bind M-RET
to new `slime-trace-dialog-copy-down-to-repl'.
(slime-trace-dialog-copy-down-to-repl): New function, in-line with
slime-repl.el's functions.
(slime-trace-dialog--format-part): Two extra text properties to
find the trace part at point.

* contrib/swank-trace-dialog.lisp (find-trace-part): new exported
function.
(inspect-trace-part): rewrite in terms os FIND-TRACE-PART.