Use virtual functions to convert NSObject to the correct type in wxDVC.

Use virtual functions to convert NSObject to the correct type in wxDVC.

Instead of trying to determine the type of the value which should be extracted
from the NSObject we receive from NSOutlineView, just pass it to a virtual
method in the renderer which knows which type does it need.

This fixes the problem with editing boolean/checkbox columns and makes the
code more elegant.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62490 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775