Don't handle branches specially when sorting items in wxDataViewCtrl.

Don't handle branches specially when sorting items in wxDataViewCtrl.

The generic version of wxDataViewCtrl tried to put all branches before the
leaves when sorting. This had a couple of problems: first, it didn't do it
correctly and actually placed the branches after the leaves which didn't make
any sense. Second, this was only done in the generic version and not the
native GTK one making the behaviour inconsistent between platforms. Finally,
this behaviour just doesn't make sense in general (i.e. when items don't
necessarily represent anything in the file system and maybe even sometimes
when they do) and there doesn't seem any good reason to do it by default.

So stop doing it, partially reverting the changes of r47562.

Closes #14100.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71425 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775