Previously it depended on addFolder() / removeFolder() calls to adjust
watchers when new folders were added or removed. There also needed to be
complex move handling.
Now, any folder creation/move-in notifications automatically trigger
watcher additions and folder deletion/move-out triggers removal.
Add state and signal to catch the following two known problems:
* Linux: inotify user watch pool is exhausted. Folder watcher becomes
unreliable.
* Windows: buffer is too small, some notifications are lost but watching
stays reliable.
