wxWebViewChromium creation is asynchronous and the object can't be
really used until OnAfterCreated() is called, so expose this in the
public API via a new event sent when the object becomes actually usable.
As a side effect, add a convenient wxWebViewEvent ctor taking wxWebView
as argument and calling SetEventObject() itself, instead of forcing all
code creating wxWebViewEvents to do it.
92f6f164b3