Improve source finding in XREF and trace-dialog buffers

* sly.el (sly--pop-to-source-location): Ensure point actually
lands at intented place.
(sly-xref-mode-map): Remove stale bindings.

* contrib/sly-trace-dialog.el (sly-trace-dialog-spec): Default
action is "show source".
(sly-trace-dialog--insert-trace): Except for the smaller "id"
button.

* contrib/swank-trace-dialog.lisp (trace-entry): New FUNCTION
slot.
(trace-arguments): Fix a bug.
(trace-location): New slyfun.
(dialog-trace): Save actual function object as returned by the
SWANK-BACKEND:WRAP interface in the trace.

* lib/sly-buttons.el (sly-button-goto-source): New button part
action.

* lib/lisp/swank-backend.lisp (wrap): Improve
documentation. Returns the saved symbol function.

* lib/lisp/backend/swank-allegro.lisp (wrap):

* lib/lisp/backend/swank-sbcl.lisp (wrap): Return the spec
arguments SYMBOL-FUNCTION. Works for now because it has to be a
symbol anyway.