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.

reve.sh

@@ -284,6 +284,11 @@ shell-completion)
   f_shell_completion
   exit 0
   ;;
+state)
+  if [ "$2" == "clean" ]; then
+    util_deleteall_temp
+  fi
+  ;;
 mode)
   in_desktop_mode="$2"
   ;;