Using new sly-button.el in sly-mrepl and sly-trace-dialog contribs

* sly-buttons.el: New file.

* sly.el (sly): Remove "Buttons" section in favour of requiring
`sly-buttons.el'.

* contrib/sly-mrepl.el (sly-mrepl): Setup M-RET slightly
differently. Insinuate into new keymaps from
sly-button.el. Insinuate into new button-type
`sly-trace-dialog-part'.
(sly-mrepl-part): New button type.
(sly-button-copy-to-REPL): New function.
(sly-mrepl-copy-to-repl, sly-mrepl--get-object-indexes-at-point)
(sly-mrepl-inspect-object-at-point, sly-mrepl-copy-down-to-repl):
Remove.

* contrib/sly-trace-dialog.el (sly-trace-dialog-part): New button type.
(sly-trace-dialog--format-part): Simplify.
(sly-trace-dialog--button): Remove.
(sly-trace-dialog--open-specs)
(sly-trace-dialog--update-progress)
(sly-trace-dialog--insert-trace): Use `sly-make-action-button'