Fixed up/down arrow keys in fullscreen preview mode and made left/right arrow keys work the same way. Also implemented the "center scroll" function, which realigns the images with the grid so there is no empty space in front of the first image.
Fixed two bugs concerning keyboard shortcuts for zoomin slider tool in lighttable, which would prevent any images from being shown (offset went outside valid range).
Replacing/moving the realign images shortcut
Moving the realign to grid functionality from the center scroll shortcut/key accelerator to a new one. This commit does it in the non-legacy way.