theme: Provide icon helper classes

- to help present icons on light backgrounds, the new fullcolor
  icons need some help. Mimic gtk styles

Needed for https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/168

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

@@ -619,6 +619,22 @@ StScrollBar {
   app menu inside the main app window itself rather than the top bar
 */
 
+/*************
+ * App Icons *
+ *************/
+/* Outline for low res icons */
+.lowres-icon {
+    icon-shadow: 0 -1px rgba(0,0,0,0.05),
+      1px 0 rgba(0,0,0,0.1),
+      0 1px rgba(0,0,0,0.3),
+      -1px 0 rgba(0,0,0,0.1);
+}
+
+/* Drapshadow for large icons */
+.icon-dropshadow {
+  icon-shadow: 0 2px 12px rgba(0,0,0,0.1),
+               0 1px 2px rgba(0,0,0,0.5);
+}
 
 /* OSD */
 .osd-window {