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/runDialog.js

@@ -21,7 +21,7 @@ const MAX_FILE_DELETED_BEFORE_INVALID = 10;
 const HISTORY_KEY = 'run_dialog/history';
 const HISTORY_LIMIT = 512;
 
-const DIALOG_FADE_TIME = 0.2;
+const DIALOG_FADE_TIME = 0.1;
 
 function CommandCompleter() {
     this._init();