Added support for wxEVT_COMBOBOX_DROPDOWN and wxEVT_COMBOBOX_CLOSEUP events to wxOSX/Cocoa.

See #15762.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75783 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775