folderIcon: Add visual drag-over feedback

WIP: This is not exactly what was discussed on IRC, but
it's looking alright as a first iteration. Design feedback
welcomed.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/664

data/theme/gnome-shell-sass/_common.scss

@@ -1514,6 +1514,9 @@ StScrollBar {
       border-image: none;
       background-image: none;
     }
+    &:drop .overview-icon {
+      background-color: transparentize($selected_bg_color,.15);
+    }
     &:active .overview-icon,
     &:checked .overview-icon {
       background-color: transparentize(darken($osd_bg_color,10%), 0.5);