Debugger: Introduce a "Native Mixed" switch

This paves the the way to mixed QML/C++ debugging
through the native backends. Currently this requires
QTC_DEBUGGER_NATIVE_MIXED to be set in the environment.

Change-Id: I126ad945e84806f3b548408318007351628c912f
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/stackhandler.cpp

@@ -64,8 +64,12 @@ StackHandler::StackHandler()
     m_contentsValid = false;
     m_currentIndex = -1;
     m_canExpand = false;
-    connect(action(OperateByInstruction), SIGNAL(triggered()),
-        this, SLOT(resetModel()));
+    connect(action(OperateByInstruction), &QAction::triggered,
+        this, &StackHandler::resetModel);
+
+    if (isNativeMixedEnabled())
+        connect(action(OperateNativeMixed), &QAction::triggered,
+            this, &StackHandler::resetModel);
 }
 
 StackHandler::~StackHandler()