Same as AbstractFieldDetails.equal, but static, so we can use libgee 0.8 without an API break.
See 673918 This can and should be removed next time we break the API. Note: This uses Gee.EqualDataFunc signature, to avoid having to cast.
| left | one AbstractFieldDetails to compare | 
| right | another AbstractFieldDetails to compare | 
| whether the elemants are equal | 
0.9.0