substantially rework file monitors

Remove all event merging and dispatch logic from GFileMonitor.  The only
implementation of GFileMonitor outside of glib is in gvfs and it already
does these things properly.

Get rid of GLocalDirectoryMonitor.  We will use a single class,
GLocalFileMonitor, for both directory and file monitoring.  This will
prevent every single backend from having to create two objects
separately (eg: ginotifydirectorymonitor.c and ginotifyfilemonitor.c).

Introduce GFileMonitorSource as a thread-safe cross-context dispatch
mechanism.  Put it in GLocalFileMonitor.  All backends will be expected
to dispatch via the source and not touch the GFileMonitor object at all
from the worker thread.

Remove all construct properties from GLocalFileMonitor and remove the
"context" construct property from GFileMonitor.  All backends must now
get the information about what file to monitor from the ->start() call
which is mandatory to implement.

Remove the implementation of rate limiting in GFileMonito...