themes: Define box-shadow for tooltips

Resolves #9795

(Solution provided by Andreas G, @uriesk)
Some system themes provide errant values for tooltip
box-shadows which cause flickering effects.
This patch defines the tooltip box-shadow to 0px to
prevent the issue.

themes/Default/common.css

@@ -473,6 +473,11 @@ tooltip box
   border: 0px solid @transparent;
 }
 
+/* Prevents flickering effect on some desktops */
+tooltip decoration {
+  box-shadow: 0px 0px;
+}
+
 /* The border around a tooltip */
 .background {
   background-color: @bg-color;