Redesigned mode-line fancyness

* sly.el (sly-modeline-string): Removed.
(sly-mode-line): New face
(sly-mode-line-format): New variable
(sly--mode-line-format): New function
(sly-modeline-state-string): Removed.
(sly-cycle-connections): Handle more cases.
(sly-dispatch-event): Update modeline more often.
(sly-selector-methods, sly-selector-other-window, sly-selector)
(def-sly-selector-method, etc): get rid of sly-selector.

* contrib/sly-autodoc.el (sly-autodoc-mode): Don't clutter
modeline with "Autodoc"

* contrib/sly-mrepl.el (sly-mrepl-mode): Don't clutter modeline
with dummy process status.
(def-sly-selector-method ?m): Removed.

* contrib/sly-scratch.el (sly-scratch): No :on-load section.

* doc/slime.texi (Top): Remove `sly-selector' node.
(Key bindings): Come up with another example.
(sly-selector): Delete.
(Multiple connections): Don't mention `sly-selector'