Linux: QFileSystemWatcher needs to gracefully deal with inotify errors

inotify has a watch limit per watch descriptor. We should deal with the case where adding a watch runs out of watch descriptors by creating a new engine (i.e. using a list of d->native instead of just a single engine).

We'd need to change addPaths engine implementation to list error codes like "try another native engine", "fall back to software polling", along with real errors, and have QFileSystemWatcher decide what to do with those errors (create a new native instance, send them to the poller, error back up to the user).