Fix freeze on WebDAV automatic sync

Hi,

I've fixed the issue #177 (closed) opened by myself.

When the automatic update was triggered, gnote check for updates on the server. It ended up trying to call GvfsSyncService::mount_sync() from the create_sync_server() method of the GvfsSyncServiceAddin class from the main thread. This method is waiting for a condition variable to be notified from the lambda function passed as a callback to mount_async which should be called by a lambda given to Gio::File::mount_enclosing_volume() in the mount_async() method.

That was never the case, because Gio::File::mount_enclosing_volume() is putting the callback in the main loop's thread pool but the main loop is stuck waiting for the condition variable to be notified in mount_sync().

Thus, I had to free this main thread by using a thread for checking for updates and emit a Dispatcher signal if any. Don't know if that's the cleanest way to do it but I've tested it and no freeze happened at all.