[LinuxCNC/linuxcnc PR#488] pyvcp: adding optional pins for spinbox’s min and max limit

This patch extends the pyvcp widget Spinbox with 2 optional pins for the limits

Example use case:
Multiple spinboxes as input for coordinates to be used in a subroutine.
Instead of manually setting the minimum limit and maximum limit of each coordinate, the settings from the machine’s ini.file can be read, such as ini.0.max_limit
Reusable interface for model series.