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/workspace.js

@@ -28,7 +28,8 @@ FRAME_COLOR.from_pixel(0xffffffff);
 
 const SCROLL_SCALE_AMOUNT = 100 / 5;
 
-const LIGHTBOX_FADE_TIME = 0.2;
+const LIGHTBOX_FADE_TIME = 0.1;
+const CLOSE_BUTTON_FADE_TIME = 0.1;
 
 const DRAGGING_WINDOW_OPACITY = 100;
 
@@ -441,7 +442,7 @@ WindowOverlay.prototype = {
         this._parentActor.raise_top();
         Tweener.addTween(this.title,
                         { opacity: 255,
-                          time: Overview.ANIMATION_TIME,
+                          time: CLOSE_BUTTON_FADE_TIME,
                           transition: 'easeOutQuad' });
     },