Add wxDataViewTextRenderer::EnableMarkup()

Implement the new method to all the implementations (generic, GTK, OS X), show
it in the sample and update the documentation.

samples/dataview/dataview.cpp

@@ -702,9 +702,14 @@ void MyFrame::BuildDataViewCtrl(wxPanel* parent, unsigned int nPanel, unsigned l
 
             m_ctrl[1]->AppendDateColumn("date",
                                         MyListModel::Col_Date);
+
+            wxDataViewTextRenderer* const markupRenderer = new wxDataViewTextRenderer();
+#if wxUSE_MARKUP
+            markupRenderer->EnableMarkup();
+#endif // wxUSE_MARKUP
             m_attributes =
                 new wxDataViewColumn("attributes",
-                                     new wxDataViewTextRenderer,
+                                     markupRenderer,
                                      MyListModel::Col_TextWithAttr,
                                      wxCOL_WIDTH_AUTOSIZE,
                                      wxALIGN_RIGHT,

samples/dataview/mymodels.cpp

@@ -436,7 +436,14 @@ void MyListModel::GetValueByRow( wxVariant &variant,
             {
                 static const char *labels[5] =
                 {
-                    "blue", "green", "red", "bold cyan", "default",
+                    // These strings will look wrong without wxUSE_MARKUP, but
+                    // it's just a sample, so we don't care.
+                    "<span color=\"#87ceeb\">light</span> and "
+                        "<span color=\"#000080\">dark</span> blue",
+                    "<big>growing green</big>",
+                    "<i>emphatic red</i>",
+                    "<b>bold cyan</b>",
+                    "<small><tt>dull default</tt></small>",
                 };
 
                 variant = labels[row % 5];