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/coreplugin/generalsettings.cpp

@@ -125,6 +125,8 @@ QWidget *GeneralSettings::createPage(QWidget *parent)
     m_page->setupUi(w);
 
     QSettings* settings = Core::ICore::instance()->settings();
+    Q_UNUSED(settings) // Windows
+
     fillLanguageBox();
 
     m_page->colorButton->setColor(StyleHelper::requestedBaseColor());