[qt5] Fix SpinMirror drawing in kalarm

The fix is basically copied from QCommonStyle. The issue appears to be
a shift in the coordinate causing the wrong edge to be exposed.
However, the whole coordinate calculation is too complex that I don't think
I know exactly what the coordinates are supposed to be.

Right-to-left rendering was tested in a hacy way by overriding option
in the CC_SpinBox rendering function.