Introduce a shell group

[leolost2605]

Since moving the panel hiding from the multitasking view completely to the shell clients and introducing a shell group are bigger changes, I think they probably should be separated which is what this PR does.

It still includes the new version of #2192 but now only introduces the shell group which is (taken from the code comment):

The group that contains all WindowActors that make shell elements, that is all windows reported as
ShellClientsManager.is_positioned_window and notifications, as well as manually added elements.
It will (eventually) never be hidden by other components and is always on top of everything. Therefore elements are
responsible themselves for hiding depending on the state we are currently in (e.g. normal desktop, open multitasking view, fullscreen, etc.).

This will allow us to fix #2130, have wingpanel and dock interactable in the windowoverview (currently they show but you can't do anything with them) or eventually we can keep e.g. the dock visible and interactable in the multitasking view.

There are no functional changes apart from the side effect that we get a nice hide animation for the panels when entering fullscreen.

This will now be required for moving all hiding logic to shell clients from the multitasking view (#2193) which will allow us to do some nice consistency improvements :)