QtIFW

Cleanup

1. Add a checkArgumentCount() helper method.
2. Fix a plural form of the check method.
3. Remove repeating translation for every UpdateOperation subclass.
4. Use always QList.at() for checking arguments,
   instead of random usage of first(), last() and at().
5. Fix const corectness.
6. Move isPersistant into #ifdef Q_OS_WIN inside
   EnvironmentVariableOperation.

Change-Id: Idaa12265f1816871de0a4ea3ae586512859a04e3
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@theqtcompany.com>