system: Enforce symbolic icons in action buttons

We can simply request the symbolic variant from CSS so that we don't
have to append '-symbolic' to all the names. This will always make
it easier to pick up that information from external sources (like
.desktop files).

https://gitlab.gnome.org/GNOME/gnome-shell/issues/433

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

@@ -1124,6 +1124,7 @@ StScrollBar {
   }
 
   .system-menu-action {
+    -st-icon-style: symbolic;
     color: $fg_color;
     border-radius: 32px; /* wish we could do 50% */
     padding: 13px;