Render groups as if they were connections if they are balancing and lack visible contents.

guacamole/src/main/webapp/styles/ui.css

@@ -448,7 +448,12 @@ div.section {
 }
 
 .group.empty > .caption .icon.group {
-    visibility: hidden;
+    opacity: 0.25;
+    background-image: url('../images/group-icons/guac-open.png');
+}
+
+.group.empty.balancer > .caption .icon.group {
+    display: none;
 }
 
 /*