Core: Implement a XOR operator for BitField

[Riteo]

At least the Wayland backend had to do a horrible backwards-and-forwards with casts to achieve a XOR operation, in our case used to get a button difference.

I'm not aware of other uses, but the core chat over RC seemed favourable at a simple operator overload.

I've never worked with core types so tell me if this is a good idea and if so, if this is implemented correctly :D

[AThousandShips]

LGTM otherwise, and makes a lot of sense

I'd maybe add more operators for completeness, but this one is specifically relevant

[akien-mga]

Thanks!