Fixes #36372 as Path2D/Path3D's `curve` property no longer uses a Curve
instance as default value, but instead it gets a (unique) default Curve
instance when created through the editor (CreateDialog).

ClassDB gets a sanity check to ensure that we don't do the same mistake
for other properties in the future, but instead use the dedicated
property usage hint.

Fixes #36372.
Fixes #36650.

Supersedes #36644 and #36656.
Co-authored-by: Thakee Nathees <thakeenathees@gmail.com>
Co-authored-by: simpuid <utkarsh.email@yahoo.com>