Save number of frames to video's metadata file

[niksirbi]

Problem

We need the frame count per video at several places in the app (e.g. ROI and event pages).
Currently, this is implemented via OpenCV thus:

vidcap = cv2.VideoCapture(video_path, apiPreference=cv2.CAP_FFMPEG)
num_frames = int(vidcap.get(cv2.CAP_PROP_FRAME_COUNT))

Since videos can be several hours long, the above calculation may take a few seconds per video, leading to delays when switching between videos.

Ideas

To improve performance, the number of frames per video should be calculated only once and saved in the video metadata file. This can be achieved by adding a "calculate number of frames" button or by silently saving the data upon first calculation.

Caveats

Alternatively, there may be a much faster way of reading the number of frames per video which I am not aware of.