Video backend selection

[Johannes Zarl-Zierl]

Hi!

I'm not yet totally pleased with the way the user is prompted to choose a 
video player backend.

So instead of prompting right away I thought that maybe just choosing a 
sensible default would be a nicer user experience (currently the first 
available of VLC > QtAV > Phonon).

Since there have been reports with some players crashing outside of 
KPHotoAlbums control, I've also added a crash detection: if KPA terminated 
during video playback, it automatically selects a different video player next 
time. As a last resort, the selection dialog is shown.

I've not yet commited the change to master, but I would like some feedback 
regarding the UX. What do you think?

The feature is currently in branch work/jzarl/crash-sentinel

https://invent.kde.org/graphics/kphotoalbum/-/commit/
78cfa242b9972792eee4661466508beed66b37aa

Cheers,
  Johannes