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/texteditoroverlay.cpp

@@ -28,6 +28,7 @@
 **************************************************************************/
 
 #include "texteditoroverlay.h"
+#include "basetexteditor.h"
 
 #include <QtGui/QPainter>
 #include <QtGui/QTextBlock>
@@ -35,8 +36,6 @@
 using namespace TextEditor;
 using namespace TextEditor::Internal;
 
-
-
 TextEditorOverlay::TextEditorOverlay(BaseTextEditor *editor)
     :QObject(editor) {
     m_visible = false;