Fix width of the wxDataViewTreeCtrl column under OSX/Cocoa.

Fix width of the wxDataViewTreeCtrl column under OSX/Cocoa.

Set the column resizing mode to "automatic" instead of "none" if the control
doesn't show the header. This allows the native control to properly resize the
only tree control column to always take up the entire window size (provided we
don't explicitly create it resizeable, so don't do this).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62602 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775