Variable to constant framerate conversion

Sometimes a video has a variable framerate, common if it's a screen recording or recorded on a phone. Variable framerate files can cause issues when exporting XML or other editor formats, because most editors expect a constant frame rate.

Recut is already able to play these files just fine, and any exported MP4 will come out in a constant framerate.

This feature would add a few explicit controls for converting from variable to constant framerate (producing a new MP4 with all audio tracks intact). It would also be nice to show info somewhere in the UI that the video is variable framerate.