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/texteditor/completionwidget.h

@@ -38,11 +38,11 @@ namespace TextEditor {
 
 class CompletionItem;
 class ITextEditable;
+class CompletionSupport;
 
 namespace Internal {
 
 class AutoCompletionModel;
-class CompletionSupport;
 class CompletionListView;
 class CompletionInfoFrame;