Allow using arbitrary windows as wxDataViewCtrl editors in wxGTK

Previously the editor created by wxDataViewRenderer::CreateEditorCtrl() had to
be a native GTK+ widget implementing GtkCellEditable interface which prevented
using composite windows (e.g. a container for a text control and a button) as
editors.

Create a helper container wrapping them now and implementing GtkCellEditable
now to allow this.

include/wx/gtk/dvrenderers.h

@@ -137,6 +137,7 @@ public:
     }
 
     virtual GtkCellRendererText *GtkGetTextRenderer() const wxOVERRIDE;
+    virtual GtkWidget* GtkGetEditorWidget() const wxOVERRIDE;
 
 private:
     bool Init(wxDataViewCellMode mode, int align);