Support for percentage formatting

Greetings,

To my knowledge, there is no way to automatically format a percentage sign according to the user's locale. As a KDE translator, this is one of the issues that is somehow inconvenient.

While translating KDE software, percent variables usually are indicated like this: %1%, %1 being the variable placeholder and the following % being the sign itself. Translators can place the percent sign however they like; in my case, Turkish, it precedes the number (%%1). Unrelated to the contents of the ticket, this causes problems with l10n parsing, so it requires some (unrelated) workarounds to properly apply this.

In other operating systems SDKs like macOS and Haiku (don't know about Windows SDK), it is possible to format this in code, and it is directly supported via ICU. There is `NumberFormatter()` in macOS and `BNumber` in Haiku. So, it's only a matter of support in the framework itself.

This will greatly enhance the i18n and the l10n experience of both developers and users. Please consider adding support for this.