state man. API && modularisation

- adds functions
  - util_create_state
  - util_read_state
  - util_write_state
  - util_delete_state
for state management API. see docs/about_states.md for details.

- moves functions from _reve.sh to _reve_utils.sh to prevent circular
imports.

chores/mode/swww_slideshow.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+# reve                              desktop environment framework
+# Yigid BALABAN <fyb@fybx.dev>                               2024
+
+# chore script
+# type      mode
+# name      swww-based wallpaper slideshow
+# desc      reruns utility/swww_slideshow when mode changes
+# vars      swwwss.dark, swwwss.light, swwwss.folder
+# reload    no
+
+# shellcheck source=../../_reve.sh
+source "$(reve where)/_reve"
+
+desktop_mode=$(util_read_config base.desktop_mode)
+
+if [ "$desktop_mode" == "dark" ]; then
+  util_write_config swwwss.folder "$(util_read_config swwwss.dark)"
+else
+  util_write_config swwwss.folder "$(util_read_config swwwss.light)"
+fi