Start adding tags

Right now, essentially nothing has changed, there is still no multi-
output support and not even a way to change/set/toggle/view/etc. tags.

However, tags *are* implemented at a core level. Next step is to add
keybinds for the various tag actions.

After that, I will work on multi-output support.

README.md

@@ -5,3 +5,8 @@ SPDX-License-Identifier: EUPL-1.2
 -->
 
 # beansprout wm
+
+
+## TODOs
+[ ] Support multiple outputs
+[ ] Support multiple seats