Add wxDataViewListModel base class for list models.

Add wxDataViewListModel base class for list models.

Introduce a base class for wxDataViewIndexListModel and
wxDataViewVirtualListModel instead of duplicating the same code in both of
them making the code difficult to maintain and change.

For now this class is not documented as it is used just to avoid duplication
in the implementation but maybe we should make it public to allow defining
other flat list data models (if this can be made to work in Carbon version).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62499 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775