fix bug #4540 #4541

bug #4540: once selected a index type, user should be able change to another index type.
bug #4541: I have removed the backend code that was forcing the auto_increment column to be part of primary key and wasn't working anyway. Instead in frontend, if user checks the auto_increment box, add index index dialog will show up if none of the index is selected.
Signed-off-by: Smita Kumari <kumarismita62@gmail.com>