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

examples/config.kdl

@@ -8,11 +8,16 @@ borders {
 }
 keybinds {
     spawn Mod4 T foot
-    focus_next Mod4 J
-    focus_prev Mod4 K
+    focus_next_window Mod4 J
+    focus_prev_window Mod4 K
+    focus_next_output Mod4+Shift J
+    focus_prev_output Mod4+Shift K
+    send_to_next_output Mod1+Shift J
+    send_to_prev_output Mod1+Shift K
     zoom Mod4 Z
     change_ratio Mod4 H +0.05
     change_ratio Mod4 L -0.05
+    reload_config Mod4+Shift R
     toggle_fullscreen Mod4 F
     close_window Mod4+Shift Q
     // Generates keybinds for keys 1-9 → tags 1<<0 through 1<<9