fix default data length setting when editing a field type

[javierariashe]

Fixed the logic for setting the default field size when changing a field’s type in the table editor.

• Moved the getUserDefaultSize and getUserDefaultPrecisionScale utility functions to a new file so they can be used when creating a new field and when editing it.
• Updated the field type change handler to use these utility functions, using the user default size and precision values.

Fixes #47

[HansMarcus01]

LGTM