Implement Pseudo Class for Dock, increase animation time for snappier reaction

This goes hand in hand with the latest Ozon Shell Theme, this will not affect User Themes unless they are adapting our new #dash:overview pseudo-class, if the User Theme doesn't have it, it just falls back to using the standard #dash class provided in the theme for Desktop Vies as well as for Overview. This reduces hacking into CSS from the extension.

Vice-versa, Ozon Shell Theme will no break stuff if you don't use Atom Dock, since the Gnome Dash never calls for a #dash:overview class it will simpy be ignored.

Theme Maintainers are encouraged (but not enforced) to support this pseudo-class for Atom Dock by simply adding something like:

#dash:overview{ /* style as seen fit */}

to their Theme.