name changes related to CMS HOOK and callers.

Renamed CMS_RESPONSE.add_..hook  with subscribe_to_..._hook
Renamed CMS_RESPONSE.call_..hook  with invoke_..._hook
Renamed CMS_HOOK_VALUE_ALTER as CMS_HOOK_VALUE_TABLE_ALTER
Renamed CMS_HOOK_MENU_ALTER as CMS_HOOK_MENU_SYSTEM_ALTER
Added new CMS_HOOK_MENU_ALTER that acts on CMS_MENU object.

Cosmetic, comments.