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/fakevim/fakevimplugin.cpp

@@ -906,7 +906,7 @@ void FakeVimPluginPrivate::triggerCompletions()
     if (!handler)
         return;
     if (BaseTextEditor *bt = qobject_cast<BaseTextEditor *>(handler->widget()))
-        TextEditor::Internal::CompletionSupport::instance()->
+        TextEditor::CompletionSupport::instance()->
             autoComplete(bt->editableInterface(), false);
    //     bt->triggerCompletions();
 }