Adding anchor line

This adds the possibility of drawing an anchor line at the begining of the print on the X axis

It introduces following parameters:
* `anchorThickness` and `anchorLinewidth`, the size of the anchor line
* `anchorSpeed`, the speed of the anchor line
* `anchorY`, the Y position of the anchor line
* `anchorFromX` and `anchorToX`, the X interval on which the line have to be

It can be disabled with anchorThickness=0