Clean up GetHBitmapForMenu.

Merge wxMenuItem::GetHBitmapForMenuCheckable and ::DoGetHBitmapForMenu
into wxMenuItem::GetHBitmapForMenu. Change its boolean parameter to an
enum specifying the kind of bitmap that is wanted (normal, checked, or
unchecked).


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76760 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775