Enhance precision of the FP conversion to strings in QVariant

This now guarantees that doing a round-trip from an FP number to string
and back to number results in the same number. This change is required
because DBL_DIG and FLT_DIG don't have the meaning that we were
expecting them to, here: they mean the minimum number of digits of
precision in decimal (i.e., changing the last decimal will always cause
the FP number to change). We need the maximum number: there is one
change in the last decimal place that causes the FP number to change.

IEEE 754 single-precision has 24 binary digits and double precision has
53 binary digits in their mantissa. To convert that to decimal, multiply
by the number of decimal digits a binary digit represents (log2(10) =
0.3), then add one for the rounding and one more digit for the actual
precision we want. That is, for floats we now ask for 9 digits and for
double, 17 decimal digits.

Task-number: QTBUG-42574
Change-Id: Ic78beb60a218f75322f832d33d63fd84e7a65b65
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Fix asymmetry in QVariant::cmp when A converts to B but not the opposite

This would result in (a == b) != (b == a). The == operation should be
commutative as much as possible.

Now, there's still an asymmetry in that b is forced to a type and the
conversion may fail. QVariant should have an idea of what conversions
are "promotion" and which ones are "demotion" (subject to loss of data
and/or can fail), so it can do the promotion first

Task-number: QTBUG-42254
Change-Id: I9fa4496bbbf0f8719ff8456cc24247290beac608
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
QVariant: Fix qIsNumericType(SChar)

signed char is a numeric type

Change-Id: I5cf9ed63cb7a2ba56c08119c77bcac0f9369b5df
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>