SpinEdit: Support of leading 0 format

The FORMAT attribute can now be used with SpinEdit widgets to support leading 0 format.

SpinEdit now accepts the FORMAT attribute (only with characters &,- and +). This allows the support of the leading 0 format. For instance, you can display 01 instead of 1:
SPINEDIT r1 = FORMONLY.r1, STEP=2, FORMAT="-&&";