Support for swaybg as a shell surface

[ddevault]

This is a lot of work for a small payoff - the mouse cursor is now
correctly rendered over the background.

Containers can now specify a custom arrange function which will be
called during arrange_windows instead of the usual path for that
container type. For backgrounds, the function sends it to the back and
sets its geometry to the size of the screen.

This requires some changes I'm not too happy with in arrange_windows
with respect to how containers figure out the arrangemnet of their
children (special cases are undesirable).

Also, if anyone runs a debug build as their actual WM, you should know
that I've made it so swaybg is invoked as ./bin/swaybg in debug builds
now.

This PR also includes a bunch of unrelated refactoring. This is a pretty
significant change and I can only test it under the x backend, so I'd
appreciate it if you kind folks would review+test it yourselves and
leave some +1s if you like it.

Also, first pull request, be nice 😉

[ddevault]

After this is merged, I'm going to implement something similar for swaybar so we can get the pointer (and pointer events) over it as well.

[ddevault]

Known issue - fullscreening a view now renders swaybar on top of it. I leave this as an exercise for another contributor to fix.

[mikkeloscar]

You can close the background with the kill command if no other windows are open.

[mikkeloscar]

Background also only seem to appear on the first workspace.

• Open sway, and see background
• Switch to workspace 2, no background.

[ddevault]

Hmm, that makes sense. I was considering a different strategy for this feature that I think I'm going to end up going with to fix that.

[Cloudef]

The recent commits in wlc allow you to now turn any surface into wlc_view https://github.com/Cloudef/wlc/blob/master/include/wlc/wlc-wayland.h#L41
So you don't have to give the background "desktop-like" role such as xdg-shell-surface or wl_shell_surface.

[mikkeloscar]

@SirCmpwn what's your ideas for this? I wouldn't mind having a crack at fixing this, but if you have some different strategy in mind, then I won't mess with this.

[ddevault]

We need to support unmanaged views that live on outputs rather than workspaces. I'd start from scratch here tbh.

[ddevault]

And it would also be better to use a custom role and use wlc_convert_to_view or whatever it's called instead of using a shell surface.

[ddevault]

Will reattempt this from scratch when the time comes.