Expanding on the implementation in d582016, add a decorator
`core.module.every()` that allows a module to specify how often to
update the module's state.
This can still be overridden using the CLI parameter `interval`.
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)
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)
Add a way for themes to specify custom separators. Doing that, make
nicer interfaces for drawing "supplementary" components (separators)
for widgets and generalize the attribute retrieval within the theme.
The core.output module now manages the list of modules and retrieves the
widgets inside draw() itself. That way, details of drawing/updating
widgets are not visible from the outside anymore.
