Enable use of TreeGrid using HierarchicalDataProvider

[jflamy]

Generic is bound to Void, which prevents use of TreeDataProvider as in the following -- which works fine with the proposed one-character fix.

        crudGrid.setCrudListener(new LazyCrudListener<TeamTreeItem>() {
            @Override
            public DataProvider<TeamTreeItem, ?> getDataProvider() {
                return new TreeDataProvider<TeamTreeItem>(new TeamTreeData());
            }
        });

[jflamy]

Only the March 15 commit is relevant (one line change) the other commits are me catching up with your code base. I have not yet mastered the art of creating clean requests I'm afraid.

[alejandro-du]

Awesome! I'll include the fix in the next release. Thanks a lot!