Enum naming on libplctag.NativeImport

[timyhac]

The enums in libplctag.NativeImport use plural form instead of singular form, which goes against naming conventions.

• DEBUG_LEVELS -> DEBUG_LEVEL
• STATUS_CODES -> STATUS_CODE
• EVENT_CODES -> EVENT_CODE

Most of the methods and objects are designed to 1-1 reflect the native API, including the constants - which is also a convention. However, the name of the group of the constants (i.e. the enum name) doesn't have a corresponding object in the native API.

This would be an API-breaking change.

Also, STATUS_CODE or just STATUS. EVENT_CODE or just EVENT