I'm not sure if we should make EditorManager responsible for tracking the mtime on behalf of custom viewers -- in particular, having this 'singletonized' as module state doesn't scale well to split views.  If there was a custom viewer base class, that might be a natural place to put it, but we don't have that at this point...  It seems cleaner to do one of these things:

* Have FileSyncManager just ask EditorManager to call something like customViewer.fileMaybeModified(mtime) 100% of the time.  And then the custom viewer could check that again whatever mtime it had last, per custom viewer instance.
* Extension of above: Actually make an (optional) custom viewer base class for tracking mtime
* Bypass FileSyncManager and listen for specific FileSystem change notifications -- like we do for updating the FindInFiles results panel.  Parsing the change events is a little tricky since there are a number of different cases (see docs at top of FileSystem module), but it's definitely the most efficient way to go -- which also will matter more once we have splitviews.