Drag select

[RobG66]

I thought one major feature missing from the datagrid was mouse drag select rows. This is available with keyboard, but has a bug where rows do not deselect when selection is reversed.

I'm not an expert and still learning... So I asked Claude to examine the (git) Avalonia treedatagrid source and add the design of that mouse select behavior to the datagrid source. The reverse selection bug was fixed as well as a separate guard statement was adjusted which was causing me trouble with something else.

Maybe someone finds it useful. Since the changes were AI and I know people don't want that code in the repositories, I'm not planning on doing anything other than using it for my own purposes. It sure beats trying to do this as a behavior!

https://github.com/RobG66/Avalonia.Controls.DataGrid

Rob