Fix typo in IsAdministrator
DragDrop: improve WPF implementation to not block ScrollBars
DragDrop: Fix all WpfDragDropHelper users to specify minimal sources

This removes all known instances of starting a drag outside of the
useful area.
ObjectReferenceViewModel: avoid accepting drops of non-assignable drops
Improve WpfDragDropHelper to use a real Control as drop target

Stuff that is not a Control cannot accept drops. this is especially true
for e.g. the ScrollContentPresenter from lists.
WpfDragDropHelper: avoid starting a drag in one control without content and snapping up content from another helper
Add another test property
WDDH: simplify self-drop check
TagPropertyEditor: avoid dragging from the popup
ZetboxListView: improve multiple selection/drag drop interaction

Now a drag can be started from a multiselection