Make QTextStream use group separators in floating-point numbers

Like for integers, this is activated only on QLocales other than C.

[ChangeLog][QtCore][QTextStream] QTextStream now uses group separators
when writing floating-point numbers when the locale is not the C locale.
The old behavior can be restored by setting QLocale::OmitGroupSeparator
on the locale.

Change-Id: Ie451b91017746c3a9b11b6211b2ddd09cd295cd2
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>