Drop DockLayer concept
With LayerShell all docks were in the above layer to match with the semantics of the specification. Under X11 our main panel was also in the above layer by setting the keep above flag.
The only thing that ended up in the dock layer were wayland applet popups, and that was mostly by accident. When they get a transient parent fixed they'll end up in the AboveLayer anyway so we should drop it now to reduce complications.
This fixes a bug where applet popups could go under the panel.