Add wxHtmlContainerCell::Detach()

This allows manipulating the HTML DOM from the outside, e.g. to detach
a header element in order to be able to repeat it on all pages.

interface/wx/html/htmlcell.h

@@ -473,6 +473,20 @@ public:
     */
     wxHtmlContainerCell(wxHtmlContainerCell* parent);
 
+    /**
+        Detach a child cell.
+
+        Detaching a cell removes it from this container and allows to reattach
+        it to another one by using InsertCell(). Alternatively, this method can
+        be used to selectively remove some elements of the HTML document tree
+        by deleting the cell after calling it.
+
+        @param cell Must be non-null and an immediate child of this cell.
+
+        @since 3.1.2
+     */
+    void Detach(wxHtmlCell* cell);
+
     /**
         Returns container's horizontal alignment.
     */