Controlling the depth slider with mouse wheel and keyboard

This is the accumulation of changes to fix:
#23: jumping depth slider on mouse wheel
#15: adding keyboard control to depth (Y)

I also changed the orientation of the depth slider to be vertically. For me it is more natural to control something like depth with an up-down slider: More up means higher layers and down deeper layers. And as most PC monitors are wider nowadays, there is also more space to the sides.
If you don't like this new orientation, please give a hint and I will implement some configuration option to let the user decide on its own.