Separate support for accounting and standard formatting of currencies

Since (maybe 5.15 but probably) 6.0 QLocale has used accountancy formats by default in currency formatting, principally because these more consistently support a distinct format for negative amounts. However, accountants' ways of representing negative amounts are not always understood by the general public, so this may be considered confusing for some use-cases (see QTBUG-89383 for examples).

Add a separate API for accounting formatting of currency amounts, revert the plain currency amounts to use the standard format and document the limitations of the latter.