Fix position issue of fullscreen preview

[JonathanTreffler]

Currently, if any parent of the gallery uses transform: translate(...) the position of the fullscreen preview gets messed up, due to an css limitation with position: fixed.

[JonathanTreffler]

I will fix this issue by using vue-simple-portal. It moves the fullscreen image into the body directly, while still preserving the vue reactivity.