Add Qt::SplitBehavior argument to QStringList::join() methods

Make a symmetric method to QString::split(), taking Qt::SplitBehavior as an additional argument. QString::split() offers to behave according to passed Qt::SplitBehavior argument - the same would make perfect sense for QStringList::join().

Use case (many places in Creator):

There is a need to join multiple error messages with '\n' separator. But sometimes returned error messages are empty strings. When forming the final error message it could be very handy to use join method. Unfortunately, when there are empty strings, they are not being filtered out, so, as a result, we may form a final error with unneeded line ends, e.g.:

void errorMessage()
{
    const QString error1 = ...; // empty
    const QString error2 = ...; // has content
    const QStringList errors = {error1, error2};
    return errors.join('\n'); // result is "\n[error2]", while desired behaviour is "[error2]"
}

Having a possibility to do:

    return errors.join('\n', Qt::SkipEmptyParts);

would be very handful.

Most probably the same could be done for QByteArrayList::join() method. Side note: it looks like QByteArray::split() is lacking Qt::SplitBehavior argument for API completeness.

In the end - not sure if the "SplitBehavior" name itself is the best in context of joining strings (as it was fine in context of splitting string). However, still the values themselves {Qt::KeepEmptyParts, Qt::SkipEmptyParts} are fine with context of joining.