Solve DC lens issues

[ZLLentz]

1. Lens motors do not converge to final position. Hypothesis is that there is a minimum speed that can be achieved through the DC drive and if the desired speed is less than that, the motor stops short.
2. Scaling is not quite right. These should go from 0 to 100 limit to limit. I think this can be achieved accurately by counting the encoder counts from limit to limit and setting the number of counts as the divisor and 100 as the numerator.
3. Homing is not set up for the user to do it easily. There needs to be an obvious "home me" button. The motor record HOMR might just work... but I need to investigate.
4. The homed position should be saved through reboots so that you can check what the zoom/focus were set to from the last boot. Deciding whether to home or not should be at the user's discretion since there is no safety concern here.