Add new Outline sidebar pane

Adds a generic Outline pane to the sidebar. The CppEditor & QmlJSEditor
plugins will implement the IOutlineWidget/IOutlineWidgetFactory interface to
provide views specific to C++/Qml/JS documents.

Reviewed-by: con

src/plugins/texteditor/texteditorplugin.h

@@ -47,6 +47,7 @@ namespace Internal {
 
 class LineNumberFilter;
 class PlainTextEditorFactory;
+class OutlineFactory;
 
 class TextEditorPlugin : public ExtensionSystem::IPlugin
 {
@@ -79,6 +80,7 @@ private:
     PlainTextEditorFactory *m_editorFactory;
     LineNumberFilter *m_lineNumberFilter;
     Find::SearchResultWindow *m_searchResultWindow;
+    OutlineFactory *m_outlineFactory;
 };
 
 } // namespace Internal