Zoom fix - Fixed arrow keys during preview and implemented center scroll  (Bug #9168)

Arrow keys (both up/down and left/right) now work during fullscreen preview. 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.

Also fixed two bugs concerning keyboard shortcuts for zooming slider tool in lighttable, which would prevent any images from being shown (offset went outside valid range).

Addition to Zoom fix for Bug #9168