Feature/csv skip rows - #4238

• Add an optional nSkipRows parameter to CSVParser::parse(). The first rows are parsed but not passed to the row function, and row numbers handed to it start at 0 after the skipped rows. A record limit counts only the rows the caller receives.
• Add a 'Skip first rows' spinbox to the CSV import dialog for files that start with a title or other information before the actual data. The value is stored with the other import settings, defaults to 0 (no change in behaviour) and is honoured by the preview, the field list generation and the import. The 'Column names in first line' option applies to the first row after the skipped ones.

Refs #1554