Make wxDataViewCheckIconTextRenderer class public

Export this class, which was only used internally by wxTreeListCtrl
before, so that user code can use it for its own columns with custom
wxDataViewCtrl models.

docs/changes.txt

@@ -136,6 +136,7 @@ All (GUI):
 - Fix calculating point position in wxDataViewCtrl::HitTest().
 - Fix position of the rectangle returned by wxDataViewCtrl::GetItemRect().
 - Add wxDataViewRenderer::GetAccessibleDescription().
+- Add wxDataViewCheckIconTextRenderer class.
 - Improve wxImage::Scale() handling of pixels with alpha channel (Tim Kosse).
 - Fix parsing of RGBA strings in wxColour (Laurent Poujoulat).
 - Refactor code in wxQuantize() for MSVC to avoid crash.