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 👍 if you like it.
