Allow using wxALIGN_CENTER in XRC for wxStaticText.

Only wxALIGN_CENTRE was allowed previously in the XRC, while both worked in
the code and also even in XRC but for wxSizer alignment.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76693 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775