Return the real column width from wxOSX wxDataViewColumn::GetWidth().

Return the real column width from wxOSX wxDataViewColumn::GetWidth().

Code used to return the last programmatically set width value instead of the
real column width which could have been changed by user if the column was
resizeable, fix this by returning the current NSTableColumn:width value.

Closes #11397.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62516 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775