operator+ API asymmetries with QStringBuilder

QStringBuilder defines operator% (and operator+ under QT_USE_STRINGBUILDER) between types that normally don't have operator+.

For the sake of API symmetry, such operator+ overloads should be added when QT_USE_STRINGBUILDER is not available.

Basically: everything I can write as a % b shall also compile as a + b, with identical semantics.

Example:

// not under QT_USE_STRINGBUILDER
QStringView s;
s + s;  // error, no such operator
s % s; // ok

Incomplete list:

• operator+(QString, QStringView) and reversed
• operator+(QStringView, QStringView)
• operator+(QByteArray, QByteArrayView) and reversed
• operator+(QLatin1StringView, QLatin1StringView)
• operator+(QLatin1Char, QLatin1Char)
• operator+(QChar, QChar)

Probably more. The last two are outright scary and probably warrant a QSB redesign. (Also, concatenating views through operator+ sounds a bit strange.)