Get tags working with actual tag-switching support

I added 7 new Commands to XkbKeybinds.Commands, though 3 of them don't
work yet.

The new commands that *work* are

+    set_output_tags: u32,
+    set_window_tags: u32,
+    toggle_output_tags: u32,
+    toggle_window_tags: u32,

They each take a 32-bit value as a bitmask to set (or toggle) tags.

The 3 unimplemented commands are

+    spawn_tagmask: u32,
+    focus_previous_tags,
+    send_to_previous_tags,

and they will all panic if they're used.

For now, default keybinds are hardcoded as part of WindowManager's
initializing in the first \`manage_start\` event. Multi-output support
is not added yet, so this will still all happen on a single Output.

README.md

@@ -10,3 +10,6 @@ SPDX-License-Identifier: EUPL-1.2
 ## TODOs
 [ ] Support multiple outputs
 [ ] Support multiple seats
+[ ] Support floating windows
+[ ] Support wallpapers
+[ ] Support a bar