1 - Restore more client properties (`icccm_focus`, `last_state`, etc.).
2 - Handle window size constraints specified by size hints.
3 - Set more attributes in `make_client` (instead of doing it in `apply_rules`) and don't pass `XCB_NONE` as argument.
4 - Internal nodes selectors/actions: labels?
6 - Use BSD `sys/{queue/tree}.h` for {list,tree} structures?