Improve value interface: return some step size
Improve value interface: return some step size

Currently Orca uses the step size to calculate
how many digits to speak.
It will actually throw an exception when we give it
0 for min step size. So any value is better.

Qt 5 patch: https://codereview.qt-project.org/#change,38807

KDE/qtatspi,KDE/qtatspi,KDE/qtatspi