[API] file kind should be of type FILE_KIND instead of uint32_t

[sudden6]

In https://github.com/TokTok/c-toxcore/blob/master/toxcore/tox.api.h#L1887 and possibly other places the kind parameter has the type uint32_t, but should have this type https://github.com/TokTok/c-toxcore/blob/master/toxcore/tox.api.h#L1638

[iphydf]

It intentionally does not have that type, because it's explicitly allowed to put your own custom values there. Clients should be able to deal with that. Unlike all other enums, the intention was that we put some common well-known values in the header, but otherwise it's free-form. Should this be changed?

[sudden6]

Ok, I didn't get this info from the docs.

If we want to keep uint32_t and some well defined values it might make sense to reserve a range for future standardized use.

EDIT: we should also define the used values explicitly then.