File system monitor imports are messy

[Manuel de la Peña]

From comments regarding the code:

I looked at the assignment to get_filemonitor_class, and poked around
a bit - some of this isn't new in this diff, but might be
nice to fix here, instead of adding code that does the same thing:
(Feel free to disagree if you think it's not productive)

# why is get_filemonitor_class being added to the namespace in platform/filesystem_notifications/__init__.py?
it looks like it's never accessed as
filesystem_notifications.get_filemonitor_class, but always as
filesystem_notifications.monitor.get_filemonitor_class, (eg in
platform/__init__.py).

So, that assignment is unnecessary, and

# importing filesystemMonitor - there are a lot of ways to import
  it. I think we could remove the assignments in platform/__init__.py,
  and filesystem_notifications/__init__.py, since it's possible to
  just get it from platform.filesystem_notifications.monitor
  everywhere - we already do this in a few places.

- syncdaemon/event_queue.py imports it from
  platform.filesystem_notifications, while test_eventqueue imports it
  from platform. (seems like those should be the same)

- test_windows and test_darwin import it from
  u.platform.filesystem_notifications.monitor.common, while test_linux
  gets it from platform.filesystem_notifications.

# related to the above: _GeneralINotifyProcessor seems like it's only
  used in test_filesystem_notifications. maybe we should change that and
  remove the assignment in filesystem_notifications/__init__.py:36