Say hello to qFloatDistance()

This function is useful if a floating point comparison requires a
certain precision. The return value can be considered as the precision.
For instance, if you want to compare two 32-bit floating point values
and all you need is a 24-bit precision, you can use this function like
this:
if (qFloatDistance(a,b) < (1 << 7)) {   // The last 7 bits are not
                                        // significant
    // precise enough
}

Task-number: QTBUG-32632

Change-Id: I020a58d2f9f9452ac3c510b4bb560dc806f0d93c
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>