Navigation sometimes 'forgets' which navigation-group is being edited (in chrome)

Defining path when setting nav_group cookie solves this problem. In chrome (and maybe other browsers) the cookie path was automatically set to '/admin', but sometimes it was set to '/admin/navigation'. The browser will choose the most close-fitting one, so the cookie with '/admin/navigation' was always seen as nav_group, while the one with '/admin' stores the most recent value: the wrong nav group was collapsed.

Also: speed up animation, feels much quicker.