Bug 342926

On reaching the Banking page, if the category combo shows that a column has been selected, and if the user decides to use that column as a memo instead, then, as intended, the user is warned of this clash.  If he/she accepts the change, then, again as expected, the category and memo combos get reset.

However, this also disables the "Next" button, even though the required columns still show as selected.  It is necessary to reselect one of the required fields in order to re-enable the "Next" button.

Patch to follow.

Reproducible: Always

Steps to Reproduce:
1.Import a CSV file and select a column to use as the category.
2.Change your mind and instead select that field to be a memo.
3.A message appears "The 'category' field already has this column selected. 
   Please reselect both entries as necessary."
4. Click OK to the message.
5. The clash results in both combos being reset, and also the "Next" button.
6. Now select that field to be a memo.
7. The Next" button is still disabled.
8. To remedy this, reselect one of the required fields - Date, Payee or Amount.

Actual Results:  
As above.

Expected Results:  
The "Next" button should not get reset if all required fields are still selected.