Add generic wxMarkupText class implementing handling of markup.

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