[ML][pipeline] Implement Pipeline::state getter

[ML][pipeline] Implement Pipeline::state getter

ACR: TWDAPI-274

[Verification] Tested in Chrome DevTools with the snippets below. Works fine

 var pipeline = tizen.ml.pipeline.createPipeline('videotestsrc ! tizenwlsink',
                  function(state) {console.log(state);})

<wait a few seconds>

pipeline.state

Change-Id: Iae5e70c63d5c9fd98877defe0949024516c8c41d
Signed-off-by: Pawel Wasowski <p.wasowski2@samsung.com>