cocoa needs a special implementation for read-only combo box 

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74175 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
now working on cocoa as well

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74176 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775