Navigate stickers with keybindings

Also minor tweaks to the way stickers are displayed.

Also update NEWS.md file.

* NEWS.md: Update with new bits about sly-stickers contrib. Add
entry about MELPA availability.

* contrib/sly-stickers.el (sly-stickers-prefix-map): New keymap.
(sly-stickers-mode-map): Just has the prefix, which is now `C-c
C-s`, not the simplest `C-c`
(sly-stickers--stickers-at): Remove useless interactive spec.
(sly-stickers--set-tooltip): Save bare message in a property.
(sly-stickers--echo-sticker): New functino.
(sly-stickers--flash-sticker): Don't flash stickers with the
'highlight face.
(sly-stickers--sticker): Assign sticker ids on creation.
(sly-stickers--arm-sticker): Not here.
(sly-stickers--briefly-describe-sticker): New helper.
(sly-stickers--sticker-modified)
(sly-stickers-delete-sticker-at-point)
(sly-stickers-maybe-add-sticker): Use it.
(sly-stickers-next-sticker, sly-stickers-prev-sticker): New
functions.