this fixes bug #1005. As a result, the Disks icon will now appear in file panels
when you change that setting with a panel open.
* _ObserverList is now in the BPrivate namespace (and renamed to ObserverList).
* its BHandler map is now only temporarily used for handlers that do not belong to
a looper yet; when BHandler::SendNotices() is called, they will be transferred
into the BMessenger map.
* Invalid messengers are now removed from the map when encountered.
* Added TODO comments about a possible reference counting if a handler is added
twice to a list (right now, this will be ignored).
* All {Start|Stop}Watching() methods are now more or less safe to be used in low
memory situations (adding some items to the map can still throw an exception...).
* Renamed BHandler::InitData() to _InitData().
* Some refactoring and cleanup.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@20029 a95241bf-73f2-0310-859d-f6bbb57e9c96
