Shorten overlay animation to 0.25 seconds

The animation time for the overlay is a bit long.  We're using 0.5 second.
From my quick tests of "another system" it takes about half that time to
animate.  The other system feels about right.

http://bugzilla.gnome.org/show_bug.cgi?id=583572

js/ui/overlay.js

@@ -46,7 +46,7 @@ const DETAILS_BORDER_COLOR = new Clutter.Color();
 DETAILS_BORDER_COLOR.from_pixel(0xffffffff);
 
 // Time for initial animation going into overlay mode
-const ANIMATION_TIME = 0.5;
+const ANIMATION_TIME = 0.25;
 
 // We divide the screen into a grid of rows and columns, which we use
 // to help us position the overlay components, such as the side panel