Previously, we would treat the input as unsigned coordinates, this is wrong!
If the coordinates are invalid, we try to find a focused or selected item.
The center of this item is where we'll show the menu.
When there is no item, we default to 0,0
Set the view object as the site of the drop target.
Implement GetItemPosition, SelectAndPositionItems, IsDropOnSource, GetDragPoint.
Use DROPEFFECT_COPY only when the item supports it.
CDefView: Make GetItemObject return an IContectMenu after setting itself as the site.
CDefViewBckgrndMenu: Make the inner context menu use the same site. Implement handling CMDSTR_VIEWLISTA and CMDSTR_VIEWDETAILSA.
CDefaultContextMenu: CMDSTR_VIEWLISTA and CMDSTR_VIEWDETAILSA shouldn't be handled in this class.
Fixes CORE-14327 and the two buttons that change the view mode
