[core] Add (partial) support for states

Add states to the modules and widgets. Widgets are mostly just a
pass-through (backwards compatibility, and ease of use - making states
directly inside the widgets would require more code inside the modules
to ensure that each widget is correctly updated).

Still missing:
- Separators during partial update (right now, it takes one interval
until separators are drawn correctly)

doc/NOTES.md

@@ -14,10 +14,10 @@
 
 ## Backwards-compatibility
 - aliases
-- charts (hbar, vbar, braille)
+- charts (braille)
 - minimize modules
 - hide modules if not in warning/error state (-a)
-- WAL support
+- WAL support / colorscheme support
 - tkinter / popups
 - scrolling decorator (incl. minwidth, alignment)
 - states