Fix QGstreamerPlayerSession::availablePlaybackRanges()

Fix QGstreamerPlayerSession::availablePlaybackRanges()

Since buffering information through buffering queries is only valid if
progressive download buffering (on-disk caching) is enabled or local
media is played and we explicitly disable on-disk buffering because of
cleanup issues with gstreamer availablePlaybackRanges() always returns
an empty range for online media.
Seeking is also possible using http range requests therefore if valid
playback ranges could not be determined we should return the [0..duration]
range unless the source is a live stream or the duration is unknown.

Change-Id: Idbebc0fdde5053c6f9c0b3794c6d122492c6239c
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>