Simplfy code dealing with the projects page

Distinguish between pages that are target specific and project specific.
Do the magic hierarchy reduction if only one target is supported just in
the projectwindow. Treat the targets page specially.

Reviewed-By: hunger

src/plugins/projectexplorer/projectwindow.h

@@ -102,7 +102,6 @@ private:
     DoubleTabWidget *m_tabWidget;
     QStackedWidget *m_centralWidget;
     QWidget *m_currentWidget;
-    IPropertiesPanel *m_currentPanel;
     QList<ProjectExplorer::Project *> m_tabIndexToProject;
 };