if seeking between keyframes

  Only text (raw UTF-8 and SSA) is supported.
  SSA tags are not properly removed yet.
* update libkate to 0.3.1

use vstream.time_base if it those values are in range, use r_frame_rate otherwise, fixes issues with some mp4 files that list field and not frame rate in the later

- disable vhooks for now, not working with ffmpeg trunk
- store and use visble height/width before initializing decoder.
  this removes green borders for some flv vp6 files.

 is not used (ok'd by j)

expose options in add video dialog,
needs cleanup to look ok but should work.

* add new option --no-upscaling to give max width, height and/or samplerate/channels without encoding at higher values than input
* fix crop