shell: Remove "Windows in Front" and "Windows Behind" visibility modes

The "Windows in Front" visibility mode is not possible to implement with
purely the layer-shell protocol as it implies that the panel is stacked
similar to xdg_toplevels.

It's possible to replicate some of its behavior by moving the panel
between bottom and above layers depending on pointer focus and screen
edge activation, but it requires some hacky code either side to make it
truly work like it used to do.

On the other hand, if you look at autohide and windows in front modes,
they are similar. The main difference is that, with autohide, the window
disappears after you leave the window, while with the latter, it stays
where it is. The screen edge is used to bring the panel to the front in
both cases.

While we could replicate the windows can cover mode, I think it's better
to remove it instead and focus on improving the autohide mode where it
seems reasonable.

The "Windows Behind" mode is removed mainly for two reasons. The first
one is that its counterpart mode "windows in front" mode has been
removed. The second one is that, in this mode, maximized and quick tiled
windows will be placed behind the panel. That's not ideal because the
window content will be occluded by the panel. Autohide or dodge active
might be better replacements instead.