Fix DetachedFromLogicalTree event args parent, and allow TreeViews to be reparented

[grokys]

What does the pull request do?

#3328 describes how a TreeView cannot currently be reparented. In particular this comment describes how the way that DetachedFromVisualTree worked made it such that it couldn't be done properly.

To fix reparenting a TreeView, I had to change the way that the DetachedFromVisualTree event works (for the better!):

• Previously when DetachedFromVisualTree was called, the Parent property of the control was still set. This meant that it wasn't possible to tell where a detachment occured. Instead set it to null before raising the DetachedFromLogicalTree event.
  • To do this, we need to store the logical root in the control so that e.g. a child control can be detached in a DetachedFromLogicalTree handler
• Add Source and Parent properties to LogicalTreeAttachmentEventArgs to give more context on what caused the attachment/detachment

Once this was in place, I was able to change TreeItemContainerGenerator.UpdateIndex to only detach the TreeViewItem from the TreeView if the logical tree detachment occurred between the TreeView and TreeViewItem - a TreeView being detached whole from the logical tree now results in it being left intact.

Checklist

• Added unit tests (if possible)?

Breaking changes

• The behavior of DetachedFromVisualTree has changed: the Parent of the detached control will now be null when the event is raised
• LogicalTreeAttachmentEventArgs' constructor now has two additional parameters (unlikely anyone is constructing one of these themselves anyway)
• LogicalTreeAttachmentEventArgs.Root is now typed as IStyleRoot not IStyleHost

Fixed issues

Fixes #3328

[MarchingCube]

LGTM. Running this change for a month now.