Added helpers for translating between wxDateTime and NSDate.

These functions will be used in the upcoming native implementations of
wxDatePickerCtrl and wxTimePickerCtrl but could also be reused elsewhere in
wxOSX/Cocoa, notably in wxDataViewCtrl where strange ad hoc code is currently
used.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70070 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775