Feature/4474 fix lightbox thumbnails

I fixed the lightbox. ;)

It now has:
- Only one row of thumbnails
- Thumbnails can be scrolled using arrows at the sides of the screen
- Thumbnails automatically scroll at a position, where the selected image will be centered, if possible
- also: fixed duplicate first image; the "largePhoto" was also shown in thumnails, causing lightbox to display it twice
I don't have a clue how to write a test for this... is there an existing test that i can copy / modify to test this?