Organize trace dialog menu and provide new "Untrace all" option

* sly.el (sly-menu): Reorganize. Tracing options moved to
`sly-trace-dialog' contrib.

* contrib/sly-trace-dialog.el (sly-trace-dialog-untrace-all): New function.
(sly-trace-dialog--shortcut-menu): Add new "Untrace all" option.

* contrib/slynk-trace-dialog.lisp (dialog-untrace-all): Return number of
specs untraced.