Added AttachedProperty IsDraggable and IsDragHandle + Logic

[NoizeDaemon]

Hey, this is my first pull request ever 😬
I'll appreciate any feedback!

Also thank you for this awesome library and all the great work you do in general for Avalonia!

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Summary

I needed more control over when a control can be dragged and where it needs to be clicked on to be dragged.

To accomplish this I introduced two AttachedProperty on CanvasDragBehavior:

• CanvasDragBehavior.IsDraggable can be set directly on the draggable Control to change it's draggability.
• CanvasDragBehavior.IsDragHandle can be set on any number of descendants of the draggable Control.
  • If IsDragHandle is not set on any descendant, the whole draggable Control acts as draghandle (current behavior).
  • If IsDragHandle is set on at least one descendant, the draggable control can only be moved by cicking those descendants.

To showcase these properties in action, I added Canvas (Advanced)-tab to the DraggableDemo-Project:

How does it work?

IsDraggable

// Set _rootControl in OnLoaded() 
_rootControl = AssociatedObject is ContentPresenter presenter ? presenter.Child : AssociatedObject;
...

// Check draggability in Pressed()
_rootControl?.GetValue(IsDraggableProperty) == true
...

IsDragHandle

The approach of setting the property directly on the descendant required moving some of the logic in OnAttachedToVisualTree() to OnLoaded(), since the GetVisualDescendants() won't get any/all descendants when called from the former.

            _dragHandles = AssociatedObject.GetVisualDescendants().Where(d => d.GetValue(IsDragHandleProperty));

            if (_dragHandles.Any())
            {
                foreach (Control dragHandle in _dragHandles)
                {
                    // Add PointerEvent-Handlers to dragHandle instead of AssociatedObject
                }
            }
            else
            {
                // Add PointerEvent-Handlers to AssociatedControl
            }

Dependency on Linq

I am using Where() and Any() in the draghandle logic, since GetVisualDescendants() returns an IEnumerable.
Should I use ToList() instead and loop over its elements to avoid the the Linq dependency, or does that not matter?