VSE: Build proxies only for slow movies

Don't build proxy file if movie is already fast enough to seek.
To determine movie performance, check if whole GOP can be decoded
in 50 milliseconds.

This test will ensure consistent performance on wide array of machines.

Downside is at most 50ms delay (per strip) when movie is added.
Ideally another test would be performed to detect whether next GOPs
do have same size or not as it may not be safe to assume that movies
will have consistent gop sizes. Even better would be to find biggest
one within small-ish range from start, and perform this test on it.
such test may add about 10ms of additional delay, but provide even more
consistency.