GUAC-1140: Use new generic down arrow instead of action-specific "open-downward".

guacamole/src/main/webapp/app/userMenu/styles/user-menu.css

@@ -127,7 +127,7 @@
     background-repeat: no-repeat;
     background-size: 1em;
     background-position: center center;
-    background-image: url('images/action-icons/guac-open-downward.png');
+    background-image: url('images/arrows/down.png');
 
 }