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/debuggerrunner.h

@@ -120,6 +120,7 @@ public:
                                  QString *errorMessage,
                                  QString *settingsCategory = 0,
                                  QString *settingsPage = 0);
+    QString idString() const;
 
 signals:
     void engineRequestSetup();