File plugin doesn't update song when file changes

When a file changes its contents (for example, by adding or removing ID3v2 tags), the File plugin should detect this and re-read the file to get the updated track data.

The file plugin does detect this; OnDirectoryEvent() is called with type=Inotify.EventType.CloseWrite. This calls AddTrack(), but AddTrack() immediately returns as the file's pathname is already known.

What needs to happen is either for AddTrack() to try calling UpdateTrack() even if the file is already known, or for the CloseWrite type action to call RemoveTrack() then AddTrack(). I've attached a patch which does the latter.