Replace wxST_MARKUP style with wxControl::SetLabelMarkup().

Replace wxST_MARKUP style with wxControl::SetLabelMarkup().

This is an incompatible change which removes the wxStaticText-specific
wxST_MARKUP style and adds wxControl::SetLabelMarkup() replacing it.

It doesn't actually change anything yet but it simplifies wxStaticText code a
lot by getting rid of many markup-related functions in it which had to behave
differently depending on whether wxST_MARKUP was used or not and also paves
way for adding markup support for the other controls in the future.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67055 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775