Debugger: Move DebuggerEngine/QmlEngine/QmlCppEngine into Debugger{}

namespace (from Debugger::Internal) as they are exported classes.
Introduce private classes to avoid exposing private JS debugger
headers.

src/plugins/debugger/watchhandler.h

@@ -40,9 +40,10 @@
 #include <QtCore/QAbstractItemModel>
 
 namespace Debugger {
+class DebuggerEngine;
+
 namespace Internal {
 
-class DebuggerEngine;
 class WatchItem;
 class WatchHandler;
 class WatchData;