dash: Add separator between favorite and running apps

Only the area used by favorite apps can be used as drop targets, it
is not possible to add new favorites between the running apps at the
end. While that behavior makes sense, it is currently impossible to
distinguish the two areas with confusing results.

Address this by adding a visual separator between favorites and
running apps.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1606>

data/theme/gnome-shell-sass/widgets/_dash.scss

@@ -30,6 +30,11 @@ $dash_border_radius: $modal_radius * 1.5;
   padding: ($dash_spacing / 2) $dash_spacing;
 }
 
+.dash-separator {
+  width: 1px;
+  background-color: transparentize($osd_fg_color,0.7);
+}
+
 // OSD Tooltip
 .dash-label {
   background-color: transparentize($osd_bg_color,0.05);