- 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.

Introduce some more private classes for the coreplugin,
use header include convention
<QtModule/QClass>, de-qobjectify VariableManager.

See BaseTextEditor::setRefactorMarkers(...). The markers support
a clicked signal via BaseTextEditor and tooltips. They feature
a cursor, but are always positioned at the end of the line (not block!).

As special gimmick they do extend the document width when they
are positioned outside the document area.