Use DoubleProperty to set AxesPanel spinboxes properly (#1093)

1. min(), max(), logRangeOrders() of AxisItem are DoubleProperty now.
2. DSpinBox is used to represent them on the panel
3. The fixed axis-range-dependent step is applied to min(), max()
4. DoubleProperty got separate useFixedStep() flag to disambiguate step type from limits type. The default state is still useFixedStep = limits.isLimited(), but can be set independently.
5. (4) => The parameter "Intensity" is formally limited (0<=..<=1e32) but now has log step

solves

• min/max fields: show in scientific format, set log step

of #1093 (closed)