general: increase pixel-size to 80 of symbolic icons in *-is-empty.ui, nautilusgtkplacesview.ui and *-no-search-results.ui to improve display clarity

Small fix, but icons in empty or "no-results" views are fuzzy (edges are not clear) since the icon size is not a multiple of 16 (the size of the symbolic icons). You can see this in an empty folder.

I've increased the icon size slightly, to 80 pixels (the left icon) so all icons appear crisp and not fuzzy. :)