Add Tree Control about eto HOT 3 CLOSED

Tree pretty much works as is, but needs more: context menu, and possible multi-column support

from eto.

For mulit-column support, the TreeView will work more like the GridView, with columns/cells defined. This is easy to do in GTK and OS X since there are built-in controls, which are based off the same implementations of the gridview.

However, the issue for Windows is there is no built-in multi-column tree for either Windows Forms or WPF. I would prefer implementations that are based off DataGridView (WinForms) or DataGrid (WPF), so as to allow editing and code sharing between tree and grid. To make them virtual, this will take some extra effort to create a sort of 'tree controller', which would keep track of expanded rows, and the depth of row ranges when things are expanded/collapsed..

from eto.

Implemented TreeGridView alongside TreeView, as TreeView will use the built-in control for windows and wpf, whereas TreeGridView uses the same control as the GridView.

from eto.