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/sourcefileshandler.h

@@ -36,10 +36,10 @@
 
 
 namespace Debugger {
-namespace Internal {
-
 class DebuggerEngine;
 
+namespace Internal {
+
 class SourceFilesHandler : public QAbstractItemModel
 {
     Q_OBJECT