Fix regression introduced by d34534b5.
Implementing a format string conversion for "%0X" correctly and in a
backwards compatible mode has proven to be harder than I thought.
So I opted to replace the deprecated QString::sprintf() with
QString::asprintf(), which is not recommended for new code but not
deprecated either.

Note: if you ever reimplement this, check the following characters for
compatibility:

 " " -> "_.20"
 "%" -> "_.25"
 "&" -> "_.26"
 "ä" -> "_.FFFFFFE4"
 "ö" -> "_.FFFFFFF6"
 "ü" -> "_.FFFFFFFC"
 "⌘" -> "_.0" //note: this is incorrect behaviour
 "\uD83D" -> "_.0" //note: this is incorrect behaviour
 "\uDE03" -> "_.0" //note: this is incorrect behaviour

The incorrectly escaped characters won't be a problem immediately,
because the attribute will be correctly mapped to the right category.
This just means that all of the last three example characters will be
mapped to the same string, leading to possible ambiguities.