parser: small optimizations to the string/char ops

- cache string.length() when used in loops (or more than once)
- use QLatin1String/QString::fromLatin1/QLatin1Char/QChar::fromLatin1
- pass more than one string in .arg(...)
- use QStringRef when comparing parts of strings
- use string.indexOf(char) instead of cycles like: while (string.at(index) != char) index++
- merge some string concatenations
- use string.chop()