stepper enable/disable

[jschoch]

it is handy to turn off the stepper to reposition. add this to the GUI and make a physical button/switch option.

[digiexchris]

also handy to turn off the stepper for manual use, my lathe's leadscrew has a handcrank at the opposite end which I do use on occasion, especially since I don't have a carriage lock. Would need a spindle resync, and should probably block the action and throw a warning if a threading mode is active. Also maybe make this a configurable parameter because some drivers are not safe to back-drive very much, high voltages can be generated by the stepper. Most of the nena23 are, but some of the 3d printer based nema17's drivers get really angry.