[panel] Use the same fade effects as menus

Currently, the buttons change their background immediately when
activated, while the menus fade in quickly - instead, make buttons
use the same effect as the menus.

https://bugzilla.gnome.org/show_bug.cgi?id=619240

data/theme/gnome-shell.css

@@ -174,6 +174,7 @@ StTooltip {
     border-radius-bottomright: 0px;
     font: 16px sans-serif;
     font-weight: bold;
+    transition-duration: 100;
 }
 
 .panel-button:active, .panel-button:checked, .panel-button:pressed {