tsdemux: Add GST_SEEK_FLAG_SEGMENT support

Submitted by Andreas Frisch @fraxinas

Link to original bug (#645584)

Description

i've narrowed the problem down to the following
a second PCR is required so that the stream bitrate can be approximated
without this variable, seek operations are not permitted (line 2565)
but the second PCR can't be recorded since the required difference of one second to the previous pcr is never reached (line 1855)
i've experimentally patched that to 3000 and voilà i can seek.
maybe somebody has a real solution for this though

when mpegtsdemux first calculates the bitrate, i'm posting a "seekable" message now so that in my application i know when i can invoke the first seek operation.
this is probably not the right way of doing this :)