The WebEngineContext member was declared after the WebContentsDelegateQt
in WebContentsAdapter. This means that the delegate was destroyed after
the context and would cause the crash.

Reorder the declaration and move the WebContentsDelegateQt also to the
private class to reduce future confusion.

Change-Id: I343504d4fd1ede80feb710446368fdf12f360b15
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>