Add multi-output support

Primary ratio is per-output.

If an output is disconnected/disabled, its windows get sent to the
previous output in the output list. If all outputs are disconnected,
windows are added to an orphan list in the WM. Once an output is
re-added, the orphans are all given to that output.

When a window is sent to a new output, it keeps the same tags as it
had before. I may add an option to take the new output's tags.

- Rename focus_next/focus_prev to focus_next_window/focus_prev_window
- Add focus_next_output/focus_prev_output
- Add send_to_next_output/send_to_prev_output commands to move windows
    between outputs

Split Seat.PendingManage.PendingFocus into separate pending output
and pending window structs

Fix window outputs when closing outputs

README.md

@@ -7,12 +7,16 @@ SPDX-License-Identifier: GPL-3.0-or-later
 # beansprout wm
 
 ## TODOs
-[ ] Support multiple outputs
-[ ] Support multiple seats
-[ ] Support floating windows
-[ ] Support wallpapers
-[ ] Support a bar
-[ ] Support starting programs at WM launch
-[ ] Support changeable primary ratio
-[ ] Support changeable primary count
-[ ] Support overriding config location
+
+These are in rough order of my priority, though no promises I do them in this order.
+
+- [ ] Support floating windows
+- [ ] Support wallpapers
+- [ ] Support a bar
+- [ ] Support changeable primary count
+- [ ] Support starting programs at WM launch
+- [ ] Support overriding config location
+- [ ] Add support for multimedia/brightness keys
+- [ ] Support multiple seats
+- [x] Support changeable primary ratio
+- [x] Support multiple outputs