Add generic wxMarkupText class implementing handling of markup.

wxMarkupText is a private class that implements generic handling of markup
strings, i.e. can measure them and render them onto a wxDC.

This class will be used for markup support in wxMSW wxButton.

Also add wxMarkupParserAttrOutput which will be useful for other wxMarkupText
implementations.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67064 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/private/markupparser.h

@@ -70,6 +70,10 @@ struct wxMarkupSpanAttributes
 // wxMarkupParserOutput: gathers the results of parsing markup.
 // ----------------------------------------------------------------------------
 
+// A class deriving directly from this one needs to implement all the pure
+// virtual functions below but as the handling of all simple tags (bold, italic
+// &c) is often very similar, it is usually more convenient to inherit from
+// wxMarkupParserFontOutput defined in wx/private/markupparserfont.h instead.
 class wxMarkupParserOutput
 {
 public: