* Remove old, unused code (TMysqlIndex, TMysqlField)
* Move relevant code from helpers to mysql_structures unit
* Rename various stuff to more compact names
* Use ordinal values instead of constants for datatypes and their categories
* Replace TGridColumn.IsXYZ boolean's by a new .DatatypeCat property
* Enhance the TComboBox used in fieldeditor to support option groups via owner drawing.
* Add some ridiculously dumb functions to translate between the dropdown's index and (non-group) item index.
* Accomplishes rfe #577.
Error message: <spatial data type> can't have a default value.
Solution: I tried every type of this family data type and found that they needs the flag HasDefault as False.