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

Backport of r75783.

Closes #15762.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75790 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775