let wxFRAME_EX_CONTEXTHELP override wxM{IN|AX}IMIZE_BOX as the latter are included...

let wxFRAME_EX_CONTEXTHELP override wxM{IN|AX}IMIZE_BOX as the latter are included in the default style and getting rid of them manually requires writing ugly wxDEFAULT_FRAME_STYLE & ~(wxMINIMIZE_BOX | wxMAXIMIZE_BOX) while this is almost always what we need

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39726 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775