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/stackhandler.h

@@ -37,11 +37,11 @@
 
 #include <QtGui/QIcon>
 
-
 namespace Debugger {
+class DebuggerEngine;
+
 namespace Internal {
 
-class DebuggerEngine;
 class DisassemblerViewAgent;
 
 ////////////////////////////////////////////////////////////////////////