(#346) Try to fix getting the wrong descendant bounds of an item (TreeViewItem) with our own GetVisibleDescendantBounds version

Credits and idea goes to @Metritutus

Fix brief deselection of selected item to be selected when changing selected item.

Fix DataGrid row anchor not being set when programmatically updating selection

Fix nested ItemsControl behaviour issues highlighted in issue #336

Better handling of UniformGrid panels

Update README to show .NET 5 compat

* Updated anchor field functionality in SetItemSelected() in ItemsControlExtensions to properly account for situations where we may not want to update the selection anchor if it's valid and there are selected items but no selected cells.

* Updated SetSelectedItem() method in ItemsControlExtensions to not deselect the item we're selecting before before selecting it again.

* Updated ItemsControlExtensions to set the DataGrid's selection anchor when the selection is programmatically changed.

* Fixed DragSourceUp() method using incorrect ItemsControl instance for item selection on mouse-up (relates to nested ItemsControls, eg RowDetails for DataGrid).
* Updated DragSourceUp() method to set the selected item for the sender object if it is an ItemsControl but is not the VisualSource of the DragInfo instance (relates to nested ItemsControls, eg RowDetails for DataGrid).

Add .Net 5 target

Disable Fody for now.

Add new attached property DragPreviewMaxItemsCount

Feature: sort drag drop preview items

Fix selection issues

Fix testing of DragSourceIgnored for an item if DragDirectlySelectedOnly is set to false

Fix DpiHelper usage