New feature: Added feature for printer movement control by keyboard

-added a new field in the control-section, for controlling the movement of
the printer by keypresses.
To control you printer by keypress, you have to connect to printer first.
Then click into the textfield to activate it. Now you can press the arrow
keys, to move the printhead in the X- and Y-directions, press the W or S
key for Z-direction or the number keys 1, 2, 3 or 4 for choose the
distance (0.1mm, 1mm....)