Fix wxDataViewChoiceRenderer behaviour in wxOSX/Cocoa.

Fix wxDataViewChoiceRenderer behaviour in wxOSX/Cocoa.

Changing the value of a "choice" cell in wxDataViewCtrl didn't work correctly
in wxOSX/Cocoa because wxDataViewChoiceRenderer used the base class version of
OSXOnCellChanged() which passed the integer index we received from NSOutlineView
to the model instead of the expected string.

Fix this by overriding OSXOnCellChanged() in wxDataViewChoiceRenderer itself
and using its argument as an integer index of the selection because this is
what it is, at least under OS X 10.7.

Closes #14373.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71939 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775