Move to the Nth next button, or Nth previous button if N is negative.
If N is 0, move to the start of any button at point.
If WRAP is non-nil, moving past either end of the buffer continues from the
other end.
If DISPLAY-MESSAGE is non-nil, the button's help-echo string is displayed.
Any button with a non-nil `skip' property is skipped over.
Returns the button found.