Refactor wxXRC to allow defining handlers outside of xrc library.

Split wxXmlResourceHandler into an ABC and the real implementation to allow
referencing the ABC in the core library itself but without pulling in all of
the XRC into it. This also allows defining XRC handlers, which only depend on
this ABC and not the xrc library, in other libraries, such as richtext, as
demonstrated by the now enabled wxRichTextXMLHandler.

Closes #10996.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72727 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/overviews/xrc_format.h

@@ -1402,6 +1402,15 @@ Example:
     Maximum length of the text entered (default: unlimited).}
 @endTable
 
+Notice that wxRichTextCtrl support in XRC is available in wxWidgets 2.9.5 and
+later only and you need to explicitly register its handler using
+@code
+    #include <wx/xrc/xh_richtext.h>
+
+    AddHandler(new wxRichTextCtrl);
+@endcode
+to use it.
+
 
 @subsubsection xrc_wxscrollbar wxScrollBar