In commit 75308a04, I added an assertion to Output.destroy() to verify
that all of its windows had been removed already. The issue is, that
only happened when Output received a .removed event, but that never
came when killing the WM directly. Instead, I simplified the handling
for the .removed event and moved most of that into destroy().
I also changed where structs have their wl_list.link removed (to inside
of their destroys) to make it more consistent. Finally,
XkbBinding.sendWindowToOutput asserts that the current output is actually
focused. Users *should* never have a focused window if there's not also
a focused output.
This commit adds support for the river-xkb-config-v1 protocol. There's
a new keyboard_layout block in config that can take options from
xkeyboard-config(7).
