Added URL support to attribute objects and to wxRichTextCtrl,

Added URL support to attribute objects and to wxRichTextCtrl,
generating a wxTextUrlEvent as appropriate.
Added outline list support and updated previews.
Added alignment support for bullets.
Added single right parenthesis support.
Added XML stylesheet reading/writing.
Changed SetBulletSymbol to SetBulletText so it can support
bullet text more generally (e.g. for cached outline list numbering)
Added wxRichTextRenderer to isolate e.g. bullet drawing and make
it customisable.
Added event handler support to wxRichTextBuffer.
Updated documentation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42431 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775