PrinterMain: make the F parameter of G1/G0 limit per-axis speed if none of the cartesian axes are specified.
DeTool.py: fix bug
DeTool.py: Make sure the meaning of the speed limit it not changed.
APrinter now implements extruder speed limit, which is in effect when no
cartesian axes are specified in G0 or G1. This commit makes sure
the truthfulness of this predicate is preserved by the
gcode transformation.