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/mainwindow.h

@@ -30,7 +30,6 @@
 #ifndef MAINWINDOW_H
 #define MAINWINDOW_H
 
-#include "core_global.h"
 #include "icontext.h"
 #include "icore.h"
 #include "dialogs/iwizard.h"
@@ -81,7 +80,7 @@ class StatusBarManager;
 class VersionDialog;
 class SystemEditor;
 
-class CORE_EXPORT MainWindow : public EventFilteringMainWindow
+class MainWindow : public EventFilteringMainWindow
 {
     Q_OBJECT