Crop to subrange of the video

Movies taken from AV recorders often have ugly letterboxes that should be removed in output videos (as every usual playback program adds it if required to fit to the screen automatically).
As far as I know FFMpeg supports cropping, so it would be nice to get this feature in OpenShot.

I propose to add an optional "output rectangle" to the preview window that is translated to corresponding options of ffmpeg.

Presets for common formats (aspect ratios like 4:3, 16:9, 16:10, ....) and resolutions would be a nice addition.