Add support for overwrite prompt of p7zip v15.09.
The overwrite prompt changed from p7zip version 9.38.1 to 15.09. Add support for the new prompt. FileExistsExpression had to be changed from QString to QStringList. Also, got rid of unneeded CliInterface::checkForFileExistsMessage(). Just use CliInterface::checkForErrorMessage() instead.
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