[core] Re-enable WAL support

Implement a generic "load keywords and replace during runtime"
mechanism, with the first concrete use-case of WAL colors (load them
during startup, and during runtime, whenever a matching name is found in
the keywords, replace with the actual color)

doc/NOTES.md

@@ -28,3 +28,6 @@
 - generalize the battery/hbar/vbar concept
 - pango output (improve - maybe autodetect? see #531)
 - allow handlers to specify whether to update or not (e.g. scroll)
+
+## TODO
+- theme: load vs. __load vs. load_keywords