Don't display error messages and try other providers when the user
cancels loading a file.
Remove files from the MRU lists if they can't be found.
Closes#717.
Originally committed to SVN as r4717.
Makes the click event handler for the grid no longer slow when video
autoscroll is enabled, making it harder to accidently select multiple
lines.
Makes seeking speed no longer limited by decoding/rendering speed;
seeking faster than video can be decoded simply results in dropped
frames.
Makes editing the file while a slow-rendering frame is visible far more
responsive.
Originally committed to SVN as r4702.
Thomas Goyne