spinner: use a 60fps spinner

- sync with gtk+ and provide a fluid spinner

https://bugzilla.gnome.org/show_bug.cgi?id=753064

js/ui/panel.js

@@ -30,7 +30,7 @@ const APP_MENU_ICON_MARGIN = 0;
 
 const BUTTON_DND_ACTIVATION_TIMEOUT = 250;
 
-const SPINNER_ANIMATION_TIME = 0.2;
+const SPINNER_ANIMATION_TIME = 1.0;
 
 // To make sure the panel corners blend nicely with the panel,
 // we draw background and borders the same way, e.g. drawing