stbt.py: Create a new video file each time video recording is restarted

The commit "Stop sink pipeline while the source pipeline is restarting"
has an issue that filesink overwrites the video file every time it gets
restarted. A possible solution would be to use the filesink in "append"
mode, but appending the next video doesn't modify the video file header,
and therefore the output file is not seekable.

A working, though slightly inconvenient solution is to save video to a
new file after every filesink restart. Running
    stbt run --save-video video.webm test.py
with a test that involves 2 source pipeline restarts will create
3 files: video.webm, video1.webm and video2.webm.
stbt.py: Save a streamable video and regenerate EBML header at the end

This reverts "Create a new video file each time video recording is
restarted" and replaces with an alternative solution.

Save a single streamable file, i.e. no indexes and duration are written,
and regenerate the header at the end to make the file seekable. As this
solution relies on `ffmpeg` that is only available in `rpmfusion` in
Fedora and Ubuntu is using `avconv` instead, this is just an
experimental commit, not ready for release.