fix compilation under ARM device.

qReal translate to different types on different architectures.
Consensus is that the current state of affairs will be kept for qt 5.0
too [1].

Luckily the function affected "SearchFieldRangeInt" is only used to
store exposure times [2] where 6 precision digit suffice.

So we add functions to write <float> and cast to <qReal> where needed
and be done with that.

[1] https://bugreports.qt-project.org/browse/QTBUG-23758
[2] ChangeLog 2008-07-07 For use with the exposuretime field ...


P.S. CustomStepsDoubleSpinBox could be converted to
CustomStepsFloatSpinBox but seem not worth the effort.
Marcel hope you're ok with this