Faster Matroska importing

[dconrad]

Importing of Matroska files takes a long time, since we have to construct a full index, and the only way to do so is to scan the entire file for a/v frames. We currently do so twice, actually. The goal would be to only read up to a little before the point in the movie that's currently playing, and support seeking so that we'd import from the keyframe before the seek to the actual seek location. We'd have to keep track of what we've imported so we don't try to import the same frames twice.

[dconrad]

(In [265]) First try at mkv idle importing. Refs #29.

[tick]

This can be something that gets completely fixed after 1.0