queue: Send the `notify` signals on queue level changes

This is documented as:

you can query how many buffers are queued by reading the #gstqueue:current-level-buffers property. you can track changes by connecting to the notify::current-level-buffers signal (which like all signals will be emitted from the streaming thread). the same applies to the #gstqueue:current-level-time and #gstqueue:current-level-bytes properties.

... but was not implemented.

This also respects the queue:silent property for the notify signals to be less intrusive.