Add ability to specify precision of float/double currency strings

Added a new overload function that allows the developer to
specify the desired precision. Until 6.0, it will require
the symbol and precision to be passed to it. Once Qt is at
version 6.0, it will replace the overload function that
requires a value and optionally a symbol.

[ChangeLog][QtCore][QLocale] Added an overload for toCurrencyString()
that allows the decimal precision to be specified.

Change-Id: I1fb7dde3583f46de2ed20ec2a7abaeca23a903ef
Task-number: QTBUG-46595
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>