[Qt] Clean up and fix coincontrol tree widget handling

• Do sorting for date, amount and confirmations column as longlong, not unsigned longlong.
• Use UserRole to store our own data. This makes it treated as ancillary data prevents it from being displayed.
• Get rid of getMappedColumn strPad - these are no longer necessary.
• Get rid of hidden _INT64 columns.
• Start enumeration from 0 (otherwise values are undefined).