Support for "motion photos"

[PhilippMundhenk]

Is your feature request related to a problem? Please describe.
Android phones can save "motion photos" which, as far as I understand are usual JPG appended with MP4 video of a few seconds before and after the shot in the JPG at an offset of exif:"Micro Video Offset" (see also here: https://medium.com/android-news/working-with-motion-photos-da0aa49b50c).
This is a neat format, summarizing both video and image into a single JPG file.

Describe the solution you'd like
I'd love to have the possibility to play the embedded video when opening the image.
Probably the video play button is a little too obtrusive, as usually the photo takes precedence here. The Windows 10 photo viewer e.g. overlays a small "motion" button top left.
I think a small play (or "motion") button in the top bar next to info could be a good solution.

This would require extraction of the video on the fly (or upfront during a library scan, though for large amounts of photos, on the fly would be preferable, if performance allows).

Describe alternatives you've considered (optional)
Of course it would be possible to extract the video of every motion photo and save it as a separate MP4 file (outside of PiGallery). This would make management of these much more tedious however, as suddenly there would be two files that are connected.