DataGrid - Add `Footer` property on the `DataGrid`

[lastunicorn]

As a user of the DataGrid,
I want to have a Footer property directly on the DataGrid (like the Title)
so that I can easily set a footer text instead of accessing it through the FooterRow and FooterCell.

Current approach:

dataGrid.FooterRow.FooterCell.Content = "Footer text";

Desired approach:

dataGrid.Footer = "Footer text";