Fade in alt-tab popup, adjust timings

The design calls for item to not appear abruptly, but not too slow
either - 100ms seems to be a good sweet spot for elements which are
supposed to appear "instantly".
Add a fade effect to the alt-tab popup and set the timings for other
fade effects to 100ms.

https://bugzilla.gnome.org/show_bug.cgi?id=621247

js/ui/workspaceSwitcherPopup.js

@@ -9,7 +9,7 @@ const Main = imports.ui.main;
 
 const Tweener = imports.ui.tweener;
 
-const ANIMATION_TIME = 0.075;
+const ANIMATION_TIME = 0.1;
 const DISPLAY_TIMEOUT = 600;
 
 const LEFT = -1;