tsdemux: Add GST_SEEK_FLAG_TRICKMODE_KEY_UNITS support

Submitted by yvonne.chen

Link to original bug (#778327)

Description

Hello
How to tickmode play ts local stream?
I find none GST_SEEK_FLAG_TRICKMODE_KEY_UNITS in tsdemux.c version 1.8.3?
Can anyone tell me how to handle fastward and reverse playback with ts stream?

Version: 1.8.2