A couple bugfixes:
* fix image-to-gcode
* fix jog wheel moves that were stopped too soon
* fix bug for the Mini GUI. Simultaneous jogging of multiple axes caused runaways.
* give the axisui hal pins correct initial values so the jogwheel axis matches the screen initially
* fix M51 (spindle speed override enable/disable)
* add "velocity mode" for jogwheels - axis stops immediately when wheel stops, even though that loses position sync between the wheel and axis
* fix occasional warning 'invalid command name "-1211088452callit"'
* fix changing axes with keyboard while jogging with the jog button