Prelight panel items on hover

In order to give feedback when the user hovers
over items on the panel, we should prelight them.

https://bugzilla.gnome.org/show_bug.cgi?id=609576

data/theme/gnome-shell.css

@@ -257,6 +257,11 @@ StTooltip StLabel {
     transition-duration: 100;
 }
 
+.panel-button:hover {
+    color: white;
+    text-shadow: black 0px 2px 2px;
+}
+
 .panel-button:active,
 .panel-button:checked,
 .panel-button:focus {