This commit renames enums related the "Curve" object type and ID type
to add _LEGACY to the end. The idea is to make our aspirations clearer
in the code and to avoid ambiguities between CURVE and CURVES.
To summarize for the records, the plans are:
- In the short/medium term, replace the Curve object data type with Curves
- In the longer term (no immediate plans), use a proper data block for 3D text and surfaces.