Hint user about apropos regexp if search fails

* sly.el (sly--last-message): New variable.
(sly-message): Set it here.
(sly-show-apropos): Display last apropos related background
message if no matches are found.
Enable multiple inspectors for sly-trace-dialog buttons

* contrib/sly-trace-dialog.el (sly-trace-dialog-part)
(sly-trace-dialog-spec): Prompt for inspector name in trace-dialog
buttons.