Make wxComboCtrlBase::Set*groundColour() methods public.

SetForegroundColour() and SetBackgroundColour() were -- presumably
accidentally -- protected in wxComboCtrlBase, even though they are
documented public virtual methods of wxWindow. This prevented their
use from user code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74607 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775