Debugger: Provide a startup customization hook for 3rd party plugins

Change-Id: Ia587abcb1bdcdfbf192d3314610ec9addf370c9b
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/debuggerengine.h

@@ -329,6 +329,7 @@ public:
     void removeBreakpointMarker(const Breakpoint &bp);
 
     QString expand(const QString &string) const;
+    QString nativeStartupCommands() const;
 
 signals:
     void stateChanged(Debugger::DebuggerState state);