Media is always set to be seekable with the gstreamer backend

Media is always set to be seekable with the gstreamer backend

Since the used mechanism for querying whether the media is seekable
was unreliable the code has been commented out and the media always
set to be seekable.
Query for seeking capabilities after the duration is known in
updateDuration() which retries to determine the duration multiple
times if unsuccessful and use gst_query_parse_seeking to check
if the media is seekable or not.

Change-Id: I141dfb1616dc59f8c92a698ddb2867f63a2656b9
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>