Fix exported headers in Core and TextEditor.

- Unexport Core::Internal::MainWindow and remove its includes.
- Move RssFetcher from Core::Internal to Core.
- Unexport CopyTaskHandler.
- Move TextEditor's completion support and Refactor Overlay
  helper classes from TextEditor::Internal to TextEditor as they
  are exported.
- Move internal BaseTextBlockSelection into private header.
- Unexport TextEditorOverlay as they are not used.

src/plugins/cppeditor/cppplugin.cpp

@@ -173,7 +173,7 @@ void CppPlugin::initializeEditor(CPPEditor *editor)
 
     // auto completion
     connect(editor, SIGNAL(requestAutoCompletion(TextEditor::ITextEditable*, bool)),
-            TextEditor::Internal::CompletionSupport::instance(), SLOT(autoComplete(TextEditor::ITextEditable*, bool)));
+            TextEditor::CompletionSupport::instance(), SLOT(autoComplete(TextEditor::ITextEditable*, bool)));
 
     // quick fix
     connect(editor, SIGNAL(requestQuickFix(TextEditor::ITextEditable*)),
@@ -400,7 +400,7 @@ void CppPlugin::quickFixNow()
             if (editor->isOutdated())
                 m_quickFixTimer->start(QUICKFIX_INTERVAL);
             else
-                TextEditor::Internal::CompletionSupport::instance()->quickFix(m_currentTextEditable);
+                TextEditor::CompletionSupport::instance()->quickFix(m_currentTextEditable);
         }
     }
 }