Includes #2192.

Same as #2150 but cleaned up for a rebase instead of squash for easier review

The additional changes here on top of #2192 are:

• Introduce a shell_group clutter actor that will be the parent of all shell UI WindowActors (e.g. panels, notifications, centered windows, etc.). This has the advantage that we have full control when and above what we show these. For example with this we can fix Can't open the applications menu when on fullscreen #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.
• Introduce a ShellClients.set_force_hide_panels to have all hiding/showing be done in the ShellClients instead of having the multitasking view do their own thing. This has to be done together with the shell group or else we get duplicate dock clones, etc.

Closes #2192 if merged before it.