Preview first 100 lines before importing

[simonw]

Importing CSV files is tricky when there's a chance they might actually be TSV, or may use an unexpected encoding or strange escaping rules.

I can address many of these issues by providing a "preview" of the CSV file before running the conversion to SQLite. The preview could operate against the first 100 rows (which can be efficiently loaded without consuming the whole file). The user can then confirm that it looks like the import is correct, or can switch to CSV or some other encoding/escaping mechanism and preview again.

This also provides the opportunity to allow the user to select the primary key, select columns that should be extracted into a separate table and select columns for indexing with FTS.