Only show the mnemonics if they need to be shown, i.e. use the same logic as
the standard controls use, to avoid showing mnemonics in wxStaticBox with
custom label colour even when other wxStaticBoxes don't show it.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65580 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775