debugger: add a 'break at main' checkbox in the 'start and debug external' dialog

src/plugins/debugger/breakwindow.h

@@ -51,6 +51,7 @@ signals:
     void breakpointActivated(int index);
     void breakpointSynchronizationRequested();
     void breakByFunctionRequested(const QString &functionName);
+    void breakByFunctionMainRequested();
 
 private slots:
     void rowActivated(const QModelIndex &index);