Enable copy-to-repl for apropos buttons and other minor hacks

* sly.el (sly-read-from-minibuffer): Don't allow empty strings
unless `allow-empty' is passed.
(sly-read-from-minibuffer-for-swank): Pass `allow-empty' to
`sly-read-from-minibuffer'.
(sly-thread-control-mode): Ensure `sly-mode' is enterer.
(sly-show-description, sly-thread-control-mode): Ensure
popup-buffer is read-only
* lib/lisp/swank.lisp (istate>elisp): Don't transmit :SERIAL while
we don't know what to do with it.

* contrib/sly-mrepl.el (sly-mrepl): Allow returning apropos
symbols to REPL.
(sly-mrepl-return): Shoosh warning.
(sly-apropos-copy-symbol-to-repl): New function.