QSettings: write keys in INI files using UTF-8

Since Qt 6 the values in INI files are always read and written as UTF-8. There was a suggestion to implement the same for the keys.

However, the keys always used %-escaped sequences for non-ASCII symbols, and never supported setting a codec.

So such change in backward-incompatible.

QTBUG-99401 implements reading keys in UTF-8, which may be useful for interaction with non-Qt applications.

The scope of this task is to implement writing of the keys as UTF-8. This can be done as a part of some new major release.