Provide means to directly iterate over QProcessEnvironment entries

QProcessEnvironment should provide a way to directly iterate over its entries.

The current way to iterate over all entries of QProcessEnvironment looks like this:

QProcessEnvironment env = QProcess::systemEnvironment();
const QStringList keys = env.keys();
for (const QString &key : keys)
    doSomething(key, env.value(key));

This is needlessly complicated and inefficient.

QProcessEnvironment should have begin() and end() which would allow us to write the code like this:

const QProcessEnvironment env = QProcess::systemEnvironment();
for (const auto &entry : env)
    doSomething(entry.key(), entry.value());