Cropping the last (milli)seconds

[fbruetting]

I thought, maybe it's useful to provide an option to crop the last hundreds of milliseconds (or seconds) at the end of the recording, because they often just contain moving the mouse to the „stop“ button. Or when the user sees, that nothing changed in the last seconds and therefore stops the recording, he/she can crop this superfluous part. This saves space and upload time, especially at higher resolutions like UHD or when big parts of the screen are captured.

[jacobbednarz]

Same deal as #97 (comment)