Add wxMarkupToAttrString helper that converts wx markup to OS X attribute
strings and use it in wxStaticText and wxButton.
This required adding new SetLabelMarkup() method to wxWidgetCocoaImpl, it
seems to make sense to have it there as it could be implemented by more
controls in the future.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67069 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
