workspace: Make previews of minimized windows translucent

Because the comment says so! Seriously, Jakub suggested this after
bug 682887 brought back a rebranded default shortcut for minimize.

https://bugzilla.gnome.org/show_bug.cgi?id=692999

js/ui/workspace.js

@@ -1207,7 +1207,7 @@ const Workspace = new Lang.Class({
 
                     // Make the window slightly transparent to indicate it's hidden
                     Tweener.addTween(clone.actor,
-                                     { opacity: 255,
+                                     { opacity: 128,
                                        time: Overview.ANIMATION_TIME,
                                        transition: 'easeInQuad'
                                      });