Add the ability to show the thumbnails

[GhaziTriki]

Showing the thumbnails like the playback does helps knowing our position in the recording. It could be made optional.

[danielpetri1]

Hi @GhaziTriki,
The exported video includes MP4 chapter marks which in essence recreate the thumnails depending on the video player being used (e.g. Apple's shows the slide images in the chapter marks, while VLC only shows a list)

Still, burning the chapter marks into the video could probably be achieved by cropping a SVG file with the thumbnails as is done with the chat.

Thanks for your suggestion!