A cancelled request is not supposed to end up in the "failed" state, but
it did, resulting in showing an error in the webrequest sample after
pressing on the "Cancel" button, which was clearly unwanted.
Don't force the application code to deal with wxObjectDataPtr<> or,
worse, calling {Inc,Dec}Ref() manually by hiding it inside the wx
objects themselves and giving the value-like semantics to them.
There should be no real changes in the behaviour, but the API does
change significantly. Notably, wxWebRequest is not a wxEvtHandler itself
any longer, as this would be incompatible with the value semantics, and
an event handler needs to be specified when creating it, so that it
could be notified about the request state changes.
This is unnecessary, it can be protected and we can initialize
wxWebRequest::m_headers directly in its ctor instead of using this
function (which also simplifies code and makes it impossible to forget
to do this).
The former can be called from the derived class ctors while the latter
only needs to be called from wxWebRequest itself, so just make it a
friend: this is not ideal, but still better than leaving this public and
simpler than any alternatives.
