refreshed many times for a single move of the Z plane.
Fixes a problem on Windows, where the new wxWidgets always returned
Cancel, even if the window was closed with the "Cut" button.
Improved performance if the live preview is disabled or not needed,
because both top and bottom parts shall be displayed.
made configurable. Now one may hide the "controller" tab and the USB/serial
connection configuration from the preferences. This is useful for someone,
who never connects his printer to the computer by a cable.