implement video query functions via ffprobe and remove moviepy dependency

[balintlaczko]

This will eliminate a potential deadlock, plus make the package slimmer and more self-contained. Since we require ffmpeg from the user (and have our suite of interfacing functions) it does not make too much sense anymore to have such a heavy dependency only for a few simple queries.

Affected functions in _utils:

• get_length
• get_framecount
• get_fps
• get_widthheight
• has_audio

[balintlaczko]

get_widthheight is now implemented.