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.