core: move ConnectionResult out of DroneCore class

[JonasVautherin]

Because it is used in many classes, ConnectionResult should not be nested into DroneCore, IMO. Moreover, having it separate makes the code a bit nicer (DroneCore::ConnectionResult becomes ConnectionResult).

Finally, I am mocking DroneCore for some unit tests and, in my situation, I need to have ConnectionResult separate from DroneCore.

[julianoes]

Usually, I used #pragma once.

[JonasVautherin]

Oh, right. I did not know #pragma once actually, and I had seen some guidelines on isocpp.github.io, saying:

Some implementations offer vendor extensions like #pragma once as alternative to include guards. It is not standard and it is not portable.

Do you have an opinion on that? I really don't know, I just read it. I can change it for a pragma if you prefer.

[julianoes]

I prefer it because it seems to work and it prevents errors when you copy and paste files without changing the define correctly.

[JonasVautherin]

ok

[hamishwillee]

To me it feels a little wrong that the ConnectionResult is not part of DroneCore, since that is the one user facing place that manages connections. I'll take your work for it that this is a good idea though :-)

I assume the header is exported for docs generation purposes?

This will require quite a few docs changes.

[JonasVautherin]

Oh, I did not think about the docs changes =/. One reason is that I want to be able to mock DroneCore from the backend. So I want to have DroneCore in production, and MockDroneCore in testing. And both are independant classes that need ConnectionResult.

And also in terms of code, it's not like if DroneCore was hiding any of ConnectionResult given that it is used in the 40 files I had to edit :D.