digiKam port to QtAV/ffmpeg instead Qt5Multimedia : add video support into Slideshow.

TODO:

- handle video rotation flag with new Exiv2 0.26 to autorotate media in player.
- port video thumbnailer to QtAV.
- remove Qt5Multimedia dependency.

For later:

- port old videoslideshow tool to QtAv.

BUGS: 159824
FIXED-IN: 5.4.0

CCBUGS: 372485
CCBUGS: 271751
CCBUGS: 361829
CCBUGS: 329854