The LeveledTreeModel case is general enough to cover
the UniformTreeModel case, so merge them and rename to
TreeModel. The former TreeModel is now BaseTreeModel.

BaseTreeModels should not be instantiated directly,
a tree model with non-uniform basic items is available
as TreeModel<>.

Done-with: Eike Ziller <eike.ziller@qt.io>
Change-Id: I64a65617ab68c0cde39cf65f4bc092ef808ee6fb
Reviewed-by: Christian Stenger <christian.stenger@qt.io>