Seeking as early as possible (before elements output data)

Submitted by Sebastian Dröge @slomo

Link to original bug (#696952)

Description

We could implement seeking in READY/NULL the following way:

• Elements that can seek will store any seek event they get before PAUSED and will automatically handle that seek event before the pre-roll
• GstBin will cache the last seek event and will send it to all elements (including elements that are added later) until the state change to PAUSED is finished (and will then remove the cached seek event)

The first part is already implemented in some demuxer, e.g. qtdemux.