If date is detected in the comments also convert it to UNIX timestamp

There is an issue when loading transactions. The date is normally loaded from the first column of transaction list and converted to UNIX timestamp format. If a date is also found in the transaction comments (in the format ddMMMyy), it is then saved into the transaction record and overwrites the original date. It is however saved in the raw textual format instead of UNIX timestamp format.
Attached diff reformats the date into "dd MMM yy" format and converts it into UNIX timestamp.
Another question is though if there is any reason to give precedence to the date found in the transaction comments over the main transaction date.