[1.6.1] Add type field for CSV import

Currently, the type of transaction is automatically detected based on the amount field (positive sign = income | negative sign = expense). However, some CSV files contain only positive amounts or even use separate columns for income and expense values.

Therefore, the CSV import should be extended with another field / section where you select the type of transaction. By default, the type is automatically calculated based on the amount, as it is now. Alternatively, you can select from the list that the type should always be treated as an income / expense.