debugger: refactor startup logic

src/plugins/debugger/script/scriptengine.h

@@ -71,11 +71,12 @@ private:
     void shutdown();
     void setToolTipExpression(const QPoint &mousePos,
         TextEditor::ITextEditor *editor, int cursorPos);
-    void startEngine();
+    void setupEngine();
+    void setupInferior();
+    void runEngine();
     void exitDebugger();
 
     void continueInferior();
-    Q_SLOT void runInferior();
     void interruptInferior();
 
     void executeRunToLine(const QString &fileName, int lineNumber);