debugger: refactoring of the plugin interface

There's  some 'external' and some 'internal' part now. Other plugins
are only supposed the external interface.

src/plugins/debugger/gdb/gdbengine.h

@@ -525,7 +525,6 @@ private: ////////// View & Data Stuff //////////
     // Convenience Functions
     //
     QString errorMessage(QProcess::ProcessError error);
-    QMainWindow *mainWindow() const;
     AbstractGdbProcess *gdbProc() const;
     void showExecutionError(const QString &message);