Feature- Allow arrow keys for pixel step movement of selected items

[MatNieuw]

On larger schematics, it is difficult to line up things using the cursor, you have to zoom in a lot and then lose the relation to things you want to align with. E.g. select one thing, and move it perfectly vertical down is difficult. If you could do the same with the cursor keys, it would be simple (although slower). See for instance the code at https://stackoverflow.com/questions/5597060/detecting-arrow-key-presses-in-javascript