Nicer compilation note navigation interface.

If point is at note beginning and last command is not a navigation
command, begin by displaying the note at point. Otherwise go on to
next note as usual. Affects `sly-button-forward' and
`sly-button-backward'.

* sly-buttons.el (sly-button--overlays-starting-at): New helper.
(sly-button--last-search-command): New variable.
(sly-button--search-1): New helper.
(sly-button-search): Use `sly-button--search-1' and
`sly-button--overlays-starting-at'