MVP of cell selection

[grokys]

Adds a cell selection mode to TreeDataGrid:

var source = new FlatTreeDataGridSource<Foo>(_data)
{
    // Columns
}

source.Selection =  new TreeDataGridCellSelectionModel<Foo>(Source)
{
    SingleSelect = false,
};

TODO:

• Runtime changing of selection model
• Ctrl+A select all not implemented for row selection either so will do it in a separate PR
• Scroll when selecting cells with keyboard
• Docs