The single `sly-stickers-dwim' keybinding needs more work.

* sly-stickers.el (sly-stickers--flash-sticker): Flash stickers.
(sly-stickers--sticker): Behave correctly when setting stickers
from region.
(sly-stickers--set-sticker-piority): New helper.
(sly-stickers--increase-prio, sly-stickers--decrease-prio): Use it.
(sly-stickers-dwim): Deactivate mark after setting from region.
(sly-stickers-operate-on-sticker): "No stickers at point" is an
error.
(sly-stickers-commit-stickers): Use a second set of overlays.
(sly-stickers-fetch): Flash all stickers, but populate only the
ones with values.

* sly.el (sly-with-popup-buffer): New :hidden option for existing
`select' parameter.
(sly-with-popup-buffer): Add edebug spec.
(sly-flash-region): Make into cl-defun, and can flash multiple times.
(sly--highlight-line): Adjust call to sly-flash-region.