Simplified generic wxDataViewCtrl's tree structure.

Simplified generic wxDataViewCtrl's tree structure.

Use just one type, wxDataViewTreeNode, to represent any kind of node.
Previously a complicated structure that represented leaves and non-leaf
nodes differently was used. This make the code way too complicated and
caused some smaller bugs (see e.g. #13256).

As a side effect, this change makes the control react correctly to
changes in IsContainer() return values.

Also fixes #13256.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68913 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775