Add a next/prev shortcut (F6 for now) for outputpanes.

This one has some magic inside to always use the current outputpane.
This solves the issue that I don't want Search and Build Issues list to
have its own set of next/prev shortcuts. todo is a better definition of
next for the build issues and/or a way to restrict it only to errors.

src/plugins/coreplugin/messageoutputwindow.h

@@ -60,6 +60,12 @@ public:
     bool hasFocus();
     void setFocus();
 
+    virtual bool canNext();
+    virtual bool canPrevious();
+    virtual void goToNext();
+    virtual void goToPrev();
+    bool canNavigate();
+
 private:
     QTextEdit *m_widget;
 };